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.