In order to understand, how many address could be generated from one seed we should go to the process of address generation in HD wallets.
As bitmover said, there are 4 billion children possible:
Might've got it wrong, but I'd like to ask the following just to make sure I understand: does this mean 2^65 is the max number of addresses your seed can create? I know the number is HUGE (36,893,488,147,419,103,232), but what happens if somebody somehow created the last possible address in their seed? Does it start over from the first address?