There is no guarantee, ever. You can do 1 reforge and get one, you can do 1000 and get none. Those what krole15 said is the correct answer: "infinity".
There's an expected average reforges, which is probably around 250, as the primal drop rate seems to be about 1 in 250 legendaries from what some people recorded only on last season in spreadsheets, IIRC. But in the end it's all RNG and "guarantee" is never gonna happen.
The only way you could ever guarantee getting one is if the droprate increased over time until it got to 100%. Even an item that drops 99.9999% of the time can never be guaranteed to drop, ever. That's how chance works.
But the droprate does not increase over time, so as others have said, you cannot guarantee it, you can only make it more likely.
Simply, the more you play, the more likely you are to see one.