ISP struggle

hello there..just out of curiosity, what does it take to be an ISP? at least how many public IP address needed to be one? and where do you buy them? IANA (internet assigned numbers authority)? or is there a distribution network? or maybe from another ISP? and what are the details on that.. (pricewise, the whole transaction)

For example, if you're allotted 100 public IP address and you had 200 customers, does that mean that only 100 connections can only log on at a time? with that in mind, is this how the ISPs do it? (SBC, Verizon, etc.?) that poses a problem doesn't it? because i've been getting logged off once in a while.

i would appreciate it whenever these questions get answered...thank you...