How many gable vents do I need for a 22x22 garage gym?

The math for the 1/150 rule puts you right at 3.2 square feet of Net Free Area (NFA) for a space that size. Those 14x24 vents usually provide around 90-95 square inches each, so four of them gets you to about 368 square inches. That is on the low side once you factor in that a gym generates way more humidity than a standard storage garage. You are looking at alot of sweat and heavy breathing in a small space. If you stick with passive, I whould aim for at least six vents or look for larger custom units to get closer to 500 square inches of total NFA. Avoid the powered gable fan if you can. Since you are installing a mini-split, you are creating a conditioned envelope. Unless your ceiling is perfectly air-sealed with spray foam or a very meticulous poly vapor barrier and canned foam on every top plate and electrical box, a powered fan will just suck your expensive cool air right out of the gym. It creates a negative pressure situation that is a nightmare for efficiency and can acutally pull moisture through your walls. If you truly have zero overhang for soffit vents, gable vents are the path of least resistance. I have seen builds where people use a hole saw to put small circular **button vents** into the frieze board between the rafters. Even a little bit of intake low down helps the gable vents actually circulate air instead of just letting stagnant heat sit in the middle of the attic. Make sure you are using baffles even if you aren't doing a ridge vent. You do not want the insulation packed tight against the roof deck at the eaves. Moisture is going to be the biggest hurdle in a home gym. If you notice any condensation or frost on the underside of the roof sheathing next winter, you will know those four vents aren't moving enought volume.

Those button vents in the frieze board sound like a great middle ground since I don't have soffits. Is that somethig an amateur can handle with a basic drill, or is it easy to mess up the wood structure?

I’m also nervous about the air sealing part. If I use standard batts and a vapor barrier, is that enough to keep the mini-split air downstairs, or do I realy need to go the spray foam route? I’d hate to waste money on a fan that just fights my AC. Also, how do you acutally find the NFA ratings? The vents at my local store don't have much info on the packaging.

You can definitly handle those button vents with a standard hole saw. Just stay centered between the rafters so you don't nick the framing. For the air sealing, batts and poly are okay, but they’re rarely as tight as they need to be for a mini-split setup. Before the insulation goes in, grab a handful of Great Stuff cans and seal every single wire penetration, top plate gap, and light box. If you skip the detail work on the ceiling plane, those gable vents will just act like a straw, sucking your cool air right out of the gym. It’s tedious work, but it’s the only way to keep your electric bill from spiking.

Don't forget about the attic access hatch itself. People spend forever sealing top plates and then just throw a piece of plywood over the hole. If that gym is going to be comfortable, you need to weatherstrip that opening and glue some rigid foam to the top of the panel. Otherwise, your mini-split is basicly trying to cool the attic through that one big gap. Also, consider the humidity factor. Even with decent venting, a heavy cardio session can turn a small garage into a swamp. You might want to budget for a standalone dehumidifier to help those gable vents out during the humid months.