Okay so source is an old engine that has been expanded over time. The first models from HL2 were ballpark 20k for heroes, 10k for characters, 1k for viewmodels and hero props, less for everything else. Textures ranged from 128x128 to 1024x1024, with a few specific textures that were 2048x2048.
That was 2004.
It’s 2014 today, and we’ve seen that source can play ball with some impressive numbers. Fakefactory, even if you hate the quality of the models, proves that the engine is able to push 100k models with multiple 4096x4096 textures; even games that are optimized for mutliplayer (CS:GO) have viewmodels that offload all detail to polycount, pushing viewmodels that are 30k triangles and 2048x2048 textures.
Source Filmmaker is capable of more - I’ve worked on projects where single frames had everything mapped at 2048x2048 to 4096x4096 textures on models that had to be assembled in-editor to bypass the roughly 100k poly-limit, we’re talking a few million polys in one image. It’s slow, but SFM isn’t a real time engine either.
It’ll likely handle whatever you throw at it, provided it’s at least targeted to real-time polycounts. I’d say that anything past 10k polygons for non-organics is wasteful, and 50k for humans is where diminishing returns start to kick in. Even that is really high, you could do things on 1/5th those numbers and still make things look good if you are decent with low poly modeling. (For hard surface, you can generally block out most shapes in 1k polys or less, it’s an artform in itself)
Short answer: As long as you’re not intentionally doing a bad job with your model, you shouldn’t really have to worry about polycount in terms of having it actually work in source. Just bear in mind that you run into problems with older model compiles at 30k polygons for a single portion of a model (i.e. the head or hair of a character) and .vtf files stop providing guaranteed rendering at 4096x4096 uncompressed, or about 40MB per map per vmt. 4096 or even 8192 is possible, provided you compress the texture - not that you should. ‘High end’ by today’s standards is really 2048x2048.
Now that said, just because you can do it doesn’t mean you should, a big part of the art of game art is not just aesthetics but also optimization.