Hey guys. I’m going to bottom paint my 30ft passenger boat just don’t know how many gallons of this bottom paint to buy. Thanks for any info, Rm

Took me about 30 seconds to find https://www.jamestowndistributors.co...int+Calculator

If it's never been previously painted you ought to barrier coat it first too . Gotta do it right the first time yanno.
Don't worry , it's only money

What product should I use for a barrier coat?

Type a barrier like type of bottom coat should be determined by boat type and location. I used Mircon CSC for years in fresh water, but would not use it in salt. Your best bet is to contact Interlux and let their folks advise on what to use

https://interlux.com/en/us/contact

All of the major bottom paint manufacturers make their own 'packages' of barrier coat systems along side their anti fouling paints. Any one of them's websites will show all of these products.
Awlgrip , Interlux , Petit are such manufacturers . I can tell you from experience that once you get your hands on the barrier coat the instructions can appear overwhelming but it's not difficult at all. You do have to start with a clean . dry and properly prepped bottom though.
They also make a big deal about the overcoat times , first coat of barrier coat to 2nd and barrier coat to first coat of anti fouling but I will confess to having skipped the chemical bond system and let everything dry and then scuff/sand the barrier coat and apply any brand of anti fouling. Once the barrier coat completely cured and dry you can pretty much apply any brand after a sanding.
It's a time consuming job though and often the day isn't long enough based on the weather as to if you can do ALL of the barrier coating and first anti foul in the same day.