How many outlets for a small recording studio/media room?
I'd say you've got the first big issue figured out, which is that you need to install an electrical subpanel and it sounds like you'll have a pro install that, so it will be properly installed (ground and neutral isolated, adequate conductors, etc). You could probably get away with a 70A service in the sub panel, but why not go for 100A, just for future capacity (appliances, tools, washing machine, charging your electric car, whatever) I wouldn't run any branch circuits inside the garage smaller than 20A, personally.

Just a tiny bit of science: a 20A circuit times 120V means that you can push a maximum of 2,400 Watts on a 20A circuit. Electrical code says you have to design not to exceed 80% of capacity with a continuous load, so that's 1,920 Watts per circuit (higher momentary draw is okay, but the breaker trips at 2,400).

My inclination is to say overindulge a little and err on the side of too many circuits instead of too few. A standard 20A circuit breaker costs $7. Cheap. However, the wet blanket is that recent electrical code changes require AFCI and/or GFCI breakers almost everywhere, and those are north of $30 each.

If you're required to use AFCI breakers, then 10 20A circuits will cost you $350 in circuit breakers alone. But you might also have some problems with AFCI breakers and some of your equipment. They aren't perfect. I think that's something you probably want to research.

You don't want things like refrigerators, A/C or fans on the same branch circuits with your recording equipment. They'll put noise on the line that your equipment will pick up.

I'd say you want at least two 20A circuits (with multiple receptacles on each) for your various amplifiers and things you might occasionally bring in like keyboards.

Ideally I'd run a 20A circuit for the projector (or future 70" 4K LED) and the high-end audio receiver and other odds and ends you'll plug in there (DVR, BluRay player, Media Computer, charging cords for phones, etc.)

I'd run a circuit for the recording console and computer(s). If you aren't playing around with a DAW now, you probably will be eventually. :-)

And you'll probably end up with a server (maybe a NAS with a bunch of hard drives) for storage and some networking equipment. Chances are that you'll also use that NAS for media streaming to the TV. So maybe two 20A circuits for the console and computer and networking equipment.

Again, all else being equal I'd say indulge a little and err on the side of more circuits than less. But those expensive AFCI circuit breakers are really kind of a wet blanket.