how to calculate internet connection speed per number of users

Depends on the nature of the business. If you are constantly going to be uploading and downloading large files, I would recommend at least T-1 for 20+ users.  On the same note, if most of the work is going to be done internally, you wouldn't really "need" more than DSL for 50 or so people.

Well here is the deal, I got 32 + remote users. An exchange server, an ACT web server, and vpn connections all the time. Currently we have a cable connection, a fast one, but everyone is screaming it's so slow ?? Was looking for some sort of calculation to get the boss to upgrade...

Any time you have incoming traffic on a cable connection, its gonna bog down the entire pipeline. With cable the bandwidth comes and goes on one channel. If you get a T-1, the incoming and outgoing traffic will operate independent of one another. I'll try to find some hard numbers to post. In the meanwhile, who is your provider and what plan do you have (if you know)?

Also:

https://www.experts-exchange.com/questions/21549514/Confusion-with-T1-vs-Cable-speeds-Looking-for-some-explanation-please.html

I understand all that. I remember when i was getting my MC-something or other. There was a calculation they had in the book for x number of users * this amount of bandwidth per user... etc...

BTW we have Armstrong Cable it's one of those mega big down but only like 100 KB up...