Read carefully the whitepaper of the company that interests you. They always write how many tokens will be released. Usually, if all tokens can not be sold, then the rest is burned
That's supposed to be done after the sale, I guess the whitepaper itself can bring things to enlightenment as we are all aware that dev team gives all the information to attract investors, its should be the first thing to do when investing reading is very essential to learn everything about the said project and learn the actual volumes and how the process to accumulate tokens.
I think the number of supply tokens is limited and only those sold during ICOs tokens are considered as supply for circulation because unsold tokens will be burned to avoid dumping of market prices.