A grain is a growth ring from a tree. So generally speaking, the more grains there are the older the tree when it was felled.
There's a lot of theories about grains, the most common being fewer grains result in more longevity, while more grains will reach their peak performance sooner (and as such break sooner).
Another thing to bear in mind is straight grains are supposed to be stronger (especially running through the toe)
Julian Millichamp says on one of his videos that 7-9 is the perfect number, and as he knows a thing or two about bats I subscribe to that theory.
That said I've never been overly fussy with grains, I think my current match bat has 13 grains and a strip of heartwood, so I wouldn't get too hung up on it. If they're evenly spaced and straight you won't go far wrong, however many there are.