Calculating the number of gates I need..??

Seems you actually mean how to estimate the resource needed for a given design. So far this is an issue of informed guess and some research.

It is interesting that this issue is never addressed by any tool makers, I guess it is quite difficult and is related to another issue: how good is the design work?

For a given functionality, some designers or tools produce tons of code and spaghetti logic while others go for better method.

I believe in this principle: There is always a better way of design, find out... For example if you need 100 counters each 14 bits you will need 1400 registers plus 100 adders ...etc. This will consume a large area of chip. But you can use an alternative, implement counters in memory blocks, find out??

Don't forget resource sharing if you can go fast.