Let's first try to explain the problem this guide is addressing, which is class balance.
Class Balance (Or Team Composition, more on that later) is basically who is on your team and what class they're playing.
Now, in a game called TEAM Fortress 2, it would be wise to be a class that helps the TEAM.
Sure, if you feel that your skill in a certain class is up to snuff, take a crack at it, if it's not, move on and try another class for a while.

Basically this

Let's give a basic example of why class balance is important.
There are three sections in a team:

• Offensive
  
• Defensive
  
• Support

Each section brings an important part of gameplay to the field.
Let's delve a bit deeper into these class sections later, but for now, I want you to visualise those three sections as part of a car.
If a car is missing (or has too much of one part while having too little in one part) a part, the car will essentially become harder to maintain.
Similar to how Team Fortress 2 works.

Now to move on to the deeper sections.

Okay, now the main reason why people play TF2 is to attack, defend, capture intel and control points.
This is where these classes are essential for consistent damage. As other classes can be used offensively but are more difficult to maintain.
The classes in this section are the easiest to use but hardest to master (depending on player to player, but design-wise, they kind of represent the three main tasks of Offensive):

• Scout (for flanking and dealing quick burst damage)
  
• Soldier (for dealing high, consistent damage that can be avoided but also to maintain an offensive (or defensive) presence on the battlefield)
  
• Pyro (for dealing ambushes and general support)

Having a lot of one class makes your team more predictable against the enemy team.
For instance, if 8 out of 12 people go Scout on your team, the enemy team can understand your attack patterns easier, as there is very few ways for Scouts to get consistent damage without flanking.
Same as Pyro, as Pyro can only deal high damage while very close to a target, so enemies will most likely stick to medium-long range combat against Pyros.
Soldier, however, is the one class where multiples are generally a good thing, as they provide high damage and coupled with their high health, they make a good choice for general front-line combat.

Basically, don't try to have too much offensive classes (except maybe Soldier).

Now every team needs some Defensive players, whether it be to protect your team's Medic(s) or to stop offensive pushes from the enemy team while your team is pushing, these classes are basically used to defend the areas that Offensive classes couldn't easily defend on their own.

Each of these classes are good at area denial, as their function is to spread fear into enemies as to prevent them from pushing.

Here's a list of what each class specializes in area denial:

• Demoman (Area denial is the Demoman's speciality, forcing players to choose alternate routes or to go to the area that the Demoman wants them to go to)
  
• Heavy (Heavy is mostly a offensive/defensive class mix, as he can do extraordinary amounts of damage but only at close range, this forces the player to do ambushes to maximize the damage he can do)
  
• Engineer (His main function is area denial but with added support for his team, such as his dispenser and teleporter)

All these classes can be played offensively, but they're not as easy to maintain their offensive stance as the Offensive classes.
Also, having too many defensive classes makes your team weaker in general face-to-face, frontline combat.
Although with multiple Engineers on tight maps with one chokepoint, it's basically an almost guaranteed win.

Now, Support classes are there to help fill in gaps that the other classes would have a very hard time to do. This section of the team has the two "Assassins" of the team and the Medic.
Each player who goes Support is exchanging frontal assault damage or extreme defensive capabilities for extra abilities that might help the team win (if done successfully).
Here are the main roles of Support:

• Medic (to provide fast mobile healing for the team and to provide a very solid backbone for pushes
  
• Sniper (to deal high damage within a short amount of time so that team mates have an easier time of finishing them off, provide area denial and to get rid of high risk targets )
  
• Spy (to kill high risk targets, kill Engineers and to confuse the enemy team)

Each Support class brings something to the battlefield that other classes can't do as easily.
Sure, a Scout could do a Spy's job, but he'll be killed quickly by the people protecting the Medic, whereas the Spy could kill the Medic and provide a distraction for his team to use.

Of course, having too many Support classes brings down your main offensive and defensive line, making it easier for the enemy team to win.

Having multiple (but not more then 4 usually) Medics, however, usually helps a team immensely.

But let's say you're faced with a dillemma like this:

You're attacking on cp_dustbowl, with 2 Snipers on your team already, but you know that they aren't contributing to the team. But you know you can do better.
You can either go Sniper, therefore weakening your offensive/defensive line or another class to provide more offense/defense.
Obviously, if you KNOW that you going Sniper/Spy/Class that isn't really good in that particular situation will help the team immensely, then by all means, go that class. If not, go to a more offensive class/defensive class.

Let me show you an example of what I mean:

This team (including myself, unfortunately) has an over abundance of Spies. This means our entire defensive line is incredibly weak and with no defense, our frontline will be pushed back.

This applies with almost all classes, as each class specializes in certain things that only that specific class can really do.

Most of all, if you don't know what class to go in a already balanced team, just use your intuition and see what class the situation calls for.


There is never a situation that calls for this

Thanks for reading a guide that I made!
Thank you TF2 Community for giving me a reason to write this guide.

Give me more reasons to write by commenting below

TL:DR: Go Medic or Soldier if you don't know what class to go.

And remember, stay classy