How many Trophies should I get before I move to TH level 8?

The number of trophies is not the basis from which you will upgrade your town hall to a higher level.
Upgrading you town hall will be most likely suitable if you already maximized most of your defensive building, storages, and etc.
Upgrading your town hall does not require a specific number of trophies.
The number of trophies however will put you in a league with players having the same range of trophies as you.
Being in a particular league will give you corresponding loot bonuses.
The higher the league you are in, the greater the bonus that you will get after every successful attack.