Nunber of PSTN lines required

Can anybody help me to determine a rough estimate for the number of PSTN lines required to connect X number of users to an other network having Y number of users?

I know it is depends on many factors, but if you take 2 average telcos in the US, what is the general rules for this?
Also, is their a site where you can read/learn more about this issue?

Thanks for any help,
B.