how many games to reach 30 with xp boost?

It depends. Winning gives more than losing, game length has an impact, as well as whether you are playing vs bots or pvp. It is hard to accurately say. XP boosts also only last a certain amount of time or a certain number of games.

Be aware though that rushing lv 30 is not worth it. Pre lv 30 you earn double the IP of lv 30 players. Since you need a certain number of champs and probably want at least two solid rune pages before playing ranked (which is the only thing lv 30 earns you) rushing to get there will short change you in the long run.

Take your time to hit 30, there is no rush and your teammates wont thank you for playing ranked the moment you hit 30.