There are two types of advisory boards: For show (i.e. credibility) or for help. They should intersect but often don't. The "For Show" advisory boards are usually no more than 5 (though I've seen up to 10) and are meant to demonstrate a significant depth of subject-matter expertise and credibility to potential customers. They act as a form of "social proof" for, meant more to be a form of "light endorsement" than to actually provide guidance.

A venture I was interim co-ceo of had such an advisory board and running formal advisory board meetings was a really big pain in that when we actually "put them to work" in asking to weigh-in on a specific question or issue, it was almost impossible to get a unified answer.

We found it better to hold quarterly events at our office which were meant to be much more social and produced some interesting conversations than formal advisory meetings.

For help advisory boards really should be no more than 3 and would advise having diverse experience but all highly relevant to the challenges you are and will be facing. I have a document I'm happy to share that spells out commitments at different levels. Just private message me your email and I can send it to you.

I recently shared this link in another related answer. I think it's relevant for your question too. Here it goes:

http://fi.co/contents/206#

Min 1 year, I recommend no less than 3, on more than 5.