There is no max to the number of accounts that a person can have. However you can only have one per email address.

Could be a few things:

The RAF offer is limited to specific countries. If you have an IP that appears to be from a country not listed in those, it can not allow you to create another RAF account.

If you create many trial accounts from the same (static) IP address, the automatic system can think that you are a spammer, creating more accounts to sit in IF or ORG and /say GO TO JULIEAS.COM FOR CHEAPEST WOW GOLD

As long as you're inside the countries where RAF is offered and you're not a spammer, the solution is to simply call blizzard, and they can set up the accounts for you no problem.