3. The "n" refers to the number of flip-flops in the counter. Just remember there are different types of counters, so "n" + the counter type tells you what the maximum count is. Most "n-stage" counters are binary, and have a maximum count of n^2, so a 7-stage 4024 counts from 0-127, a 12-stage 4040 counts from 0-4095, etc. Same for gray code and other binary counter variations. Max count is the same whether the counter is ripple or synchronous.
A 4-stage decade counter is a binary counter that wraps around at a count of 9 instead of 15. Johnson type counters have a max count of 2n, so a 5-stage 4017 counts to 10. Linear feedback shift registers have n^2 - 1 states.