I believe it was planned to have six games, but only ended up being three (whether it was due to sales, creative disputes, or other reasons, I'm not really sure on the specifics).

Either way there are only three Xenosaga games out atm, I'm not sure if they're going to make another one. But if you wanted to play more Xeno games you could try Xenosaga (PS1) which is sort of a spiritual prequel.