For game assets it varies, how ever, generally less the better. Again though it varies, what the asset is meant to be, if its hero prop for cinematic and focused only on, 800k might be good for it, if it runs 60fps. then if you have props that you have 200 on the screen at the same time, then the even the 10k would be too much. it all depends what the purpose is. and again don’t get hung up on the numbers, 200 10k objects might be still good. moderm game engines eat polygons good nowadays, what you have to be aware is to run profiling tools in them to see if you are running into problems, if not, then you are good to go.
The actual retopo stuff and what to take into count, Check the sniper rifle course, If i remember correctly it had portion that explained how not to waste vertices and keep only form.
