Between 15-20 lbs. You can get 15lbs of base rock and 5 or so lbs of LR
if youre budget friendly. If you have the cash to buy 20lbs of lR
that would be fine too.
The base rock will become live over time as you cycle so you can save quite bit of money.
I would recommend that you either use a shrimp or pure ammonia to help your cycle, makes it nice and easy and quick in my opinion. You can dose ammonia each morning and test to see when it has converted to nitrites then to nitrates. It could take a week or 3 just depends.
If you don't have it already I would strongly urge you to pick up a test kit for your water. API master saltwater kit has all that youll need to cycle and its about $25 give or take.