I've run this basic setup for over 8 years. Started out with 2 red Blitz cans. During my first crossing of the Mojave Road, a lot of fuel spilled on the tuck from the CARB caps. After that I've picked up some NATO style cans. I've never had a single issue with leaks, even though the gaskets should be replaced on my current cans. The mount is from Front Runner. It's adjustable to fit cans that aren't exactly to the NATO standard and have accommodations to fit a pad lock. It's mostly for security of the latch itself, not from would be gas thieves.
How much fuel to carry varies by a lot of factors. If you're in Hi or Lo range, hard/soft terrain, available fuel stops, size of tank, etc. I typically carry two cans for anything more than a day trip. If it's not used on the trail, it'll get dumped into the tank on the way home. The mount is for dual cans so it's always multiples of two for me. Sometimes I used to do the water can and a gas can up there, but wanted faster access to the water, so it's inside the cargo area now.
On a week long trip to Death Valley, I carried x4 Jerry Cans. Even if you can get fuel, you may be paying twice as much for it. Just don't let them see you filling from your cans instead of buying their gas ;) But depending on your situation, you may well need to refill the cans with the expensive gas anyways. And in reality, I usually don't mind paying the increased price for local gas. If it helps keep that station open, or that little town around for the next time I'm there, it's ok with me.
My best tips if you carry fuel on the roof are these.
Empty your cans ASAP. Don't wait days until you're down to a 1/4 tank to dump the Jerry cans. That whole time you're driving, you're reducing the weight down low by consuming gas, but that 60 lbs. of fuel (plus the weight of x2 cans) continues to sit up high on the roof.
Use a Super Siphon. No sense in unloading full cans off the roof, just to hold them while pouring fuel into the tank. Stick the open end of the Super Sipohn into the filler neck, and the rattly end into the can. After a couple shakes you'll have free flowing gas dumped into the tank in no time.
Store the cans near the filler neck. You don't want to lug full cans from the front to the back of a loaded roof rack. Some mounts won't allow a Jerry Can to stand upright inside the lower holder, so you'll need space to set the can upright elsewhere.