This error occurs when the editor can't render every part of the map due to too many entities within the Source engine. The game puts it simply as "too many items," because saying "too many entities" could be confusing to those not familiar with the Source engine and the terms involved with it.

For those not familiar, basically every item in the editor that is not a wall, floor, or ceiling is known as an entity. This means the game has to load not just what the object looks like, but how it works. (How it moves, effects the players or objects or environment, etc.) This extra loading makes the game engine work a little harder to keep track of everything in the map and keep it running. Too much information to handle from too many entities can make the game unstable and cause a crash. Therefore, Valve puts an entity limit so this should not happen when your map is created.

The good news is the amount of entities allowed is a generous 1750 per map. The bad news is that a single item is comprised of multiple entities. So it's not a matter of 1750 individual items per map, but 1750 parts of those items per map. That's where this guide comes in! You should find the entity number of any and all items within this guide to help you replace the more entity heavy objects with smaller ones, hopefully allowing you to render your map!

-Each category in this guide list entities of the specified type and there cost.
-At the bottom is a list of every entity and it's cost from highest to lowest.

Pedestal Button
Entity Cost: 6

Weighted Button
Entity Cost: 3

Cube Button
Entity Cost: 3

Sphere Button
Entity Cost: 3

-Note: Remember that in addition to the button cost, it cost more entities to have cubes and spheres around the map, and cost even more if those items have droppers. Try to rely on Pedestal Buttons more if the cost is getting too high.

-Note: The indicator lights that show connections from buttons to items are entities as well. You can set the connection visibility to "None" if the entity amount is getting a little high. Just be sure that it doesn't make your puzzle too confusing omitting those.

Cube Dropper
Entity Cost: 33
-Note: Cube Droppers are the 2nd most entity heavy item in the game, so try to minimize the amount you have. If your test doesn't have a way to destroy or lose a cube, you can remove the dropper to save a lot of space.

Weighted Storage Cube
Entity Cost: 4

Weighted Companion Cube
Entity Cost: 4

Edgeless Safety Cube
Entity Cost: 4

Reflection Cube
Entity Cost: 4

Franken Cube
Entity Cost: 4

-Note: All cubes cost the same amount of entities, so use your favorite or what ever is most fitting for your test!

Glass Panel
Entity Cost: 12

Angled Panel
Entity Cost: 13

Flip Panel
Entity Cost: 8

Stairs
Entity Cost: 13

Glass
Entity Cost: 1 Per Block
Note: The cost of Glass is 1 entity for every space it occupies. Meaning stretching the glass up or to the sides will increase the cost by 1 for each space you do this on.

Grating
Entity Cost: 2 Per Block
Note: Just like Glass, the cost of Grating goes up for every space it occupies. Meaning stretching the grating up or to the sides will increase the cost by 2 for each space you do this on. This means it is more cost effective to use Glass in place of Grating where acceptable for the test.

Tractor Beam
Entity Cost: 8

Faith Plate
Entity Cost: 9

Light Bridge
Entity Cost: 4
-Note: A Light Bridge cost 4 entities regardless of length.

Piston Platform
Entity Cost: 44
-Note: Piston Platforms cost 44 entities regardless of length. They are also the most entity heavy item in the game! Try to use Faith Plates, Tractor Beams, or let the player portal themselves to move from one place to another when the entity cost is getting too high.

Track Platform
Entity Cost: 13+1 Per Block
-Note: The cost for Track Platforms are 13 for the initial space, then 1 entity per length of track used.

Gel Dropper
Entity Cost: 15

Repulsion Gel
Entity Cost: 8

Propulsion Gel
Entity Cost: 8

Conversion Gel
Entity Cost: 9

-Note: Since having 2 or more of any Gel puddle lying around cost more entities than a Gel Dropper itself, it is more cost effective to have players be able to paint the level themselves from a dropper when possible.

Cleansing Gel
Entity Cost: 8
-Note: This Gel instantly disappears when it comes into contact with the environment. As such, it is a waste of entity space to leave Cleansing Gel puddles around the level.

Laser Emitter
Entity Cost: 6

Laser Catcher
Entity Cost: 4

Laser Relay
Entity Cost: 3

Reflection Cube (Repeated from Cube Entities section)
Entity Cost: 4

Fizzler Field (A.K.A. Material Emancipation Grill)
Entity Cost: 14+9 Per Block
-Note: Fizzler Fields cost 14 for the initial placement, regardless of it's length. Then for every space you stretch it to the sides it cost 9 more entities, regardless of length.

Laserfield
Entity Cost: 14+9 Per Block
-Note: Just like the Fizzler Fields, it cost 14 for the initial placement regardless of it's length. Then for every space you stretch it to the sides it cost 9 more entities, regardless of length.

Turret
Entity Cost: 3

Deadly Goo
Entity Cost: 1 Per Pit
-Note: Deadly Goo only cost 1 entity, regardless of the pit's size. This is the lightest entity cost in the game!

Light Strip
Entity Cost: 3

Small Observational Room
Entity Cost: 2

Large Observational Room
Entity Cost: 0

Basically when you find yourself needing to remove some items to make your map render, try to aim for items at the top of this table first to save the most of your map.

I hope this guide helps! At the very least it should be an easy way to access entity cost while in game.