That was easy.
Seriously, though, there's no simple number that I can give you. However, there are signs that give you a good idea as to whether or not you are ready to get married. I'm going to make it easy and give you a pair of lists that have all of the signs you need.
You should get married if:
-You can pay for it yourself or your parents have made it clear that they want to pay for it.
-You really, truly love the man (or woman) and can imagine a long and wonderful future with him (or her).
-You are both in agreement about children. Seriously, be in agreement about children.
-You both have similar ideas about where you want to live (part of country, city vs. suburb vs. rural).
-You can tolerate your in-laws.
-You never go to sleep angry at the other person.
You shouldn't get married if:
-It will put you into debt.
-Everyone on Earth but you thinks it's a horrendous idea.
-You definitely want children and he definitely doesn't, or vice versa.
-You really like your boyfriend, but aren't sure you love him.
-If you feel as if you have to because society/your friends/your family is putting pressure on you.
-You have very different plans for your future.
-You're constantly fighting over big, important things like money or where to live.
These two lists aren't, of course, exhaustive, but I think they get my points across. If marriage will make your and your boyfriend's life better, then get married. If you're not yet there, then wait. You don't get bonus points for marrying early, and you don't get any penalties for marrying late. Do what's best for you and your boyfriend. That's your first, and only, priority.