Number of specs on cloud per user

i have a questions about the right way to calculate the specs need for 10 to 50 user per one site (more than 50 site ) every site contain 10 to 50 user using multi tenet.
so how i calculate the right cloud and right specs to be fair.
2- does every site have his own worker /threads configuration
specs need to determine>
1- no of instances
2- no GB ram
3- no of cores
4 - size of storage