polycount IOS / Android games?

there's lots of information on this online. But it depends on the device you are targeting. For instance the old 3G iphone is much less powerful than newer devices. In general read up on the docs provided by Unity. As a general rule of thumb keep your poly counts low... very low. The max # of vertices you should have on screen at any point in time should be under 50k for iphone 3GS +... under 10k for 3G... but that's only part of it. lots of other elements can slow down game play. You should really read the getting started sections of unity in the docs... specifically for mobile devices. That's what I did. Unfortunately I did this after I began working on the project so I had to go back and re-do a bunch of things. I remember one of my original models had 68k vertices haha... now he has 600. lol... read up on everything, sound, hardware... everything! the more you know the better the performance you will have!

The rough answer is, you can as many as the devices can support. You want to use as few as possible.

This is especially true for Android, where there is massive variance between the capabilities of devices. So your best course of action is to think, the lower the better. I may be wrong on this but what I remember before I started modelling my character was DS games had 300-400 vertices per char, mobile games can have 800-1000 verts, and modern console games have around 6000-7000. With that many characters on screen, lower is best, I'd recommend seeing what you can do with 200-300. Also, for tower defense, I'd think you could get away with 64x64 textures, definitely 128x128. Try different sizes of textures on your model, back away from the screen until the size is about the same as you'd expect on the game, and go with the lowest texture size that looks decent. Google searches for low-poly models and browsing through Google SketchUp can help tremendously as far as inspiration goes. See if you can find what Anomaly: Warzone used. That game looked and played great.