Your questions are very broad. I'll give you high-level answers that I think should help you understand the breadth of the subject.
1. At last count, I had enrolled in 8 airline programs, 9 hotel programs, and 3 rental car programs. In each of these categories I am generally high-level elite in 1 program, mid-level in 1 or 2 others, and low- or no-status in the rest.
2. I enrolled in each of these programs because a) the potential benefits were valuable, and b) I foresaw traveling with each brand frequently enough that I would be able to realize that value. Understand that the numbers in my previous answer are low because of alliances and partnerships that allow me, for example, to rent cars from 8-10 different companies and earn valuable points with airlines instead of having to manage accounts with 8-10 separate car companies.
3. I travel for both work and leisure. At peak I was flying in excess of 150,000 miles per year. That helps explain the large number of programs I've enrolled in.