How many gallon air tank to fill a kite.
Don't get air tank but a helium tank. Your kite will fly better..
But seriously about the topic - electric compressor. If you need to carry
the electric compressor to the beach it most likely takes you more energy than pumping. Typical kiters I see carry two-three kites, 2 boards to shore line. Add there the electric pump, another 5-6kg bag. My friends have a couple of BST electric pumps but they never ever bother to carry it.
Considering air tanks. You probably cannot self-fill 200bar scuba tanks. 135psi cheap compressor equals to about 9 bars. 9bars on a 10l tank is 90l of air. If kite requires roughly 200l of air and you want to be able to fill 5 kites, with 9 bars to get 1000l of air means 111l tank = 29 gallons (of some US gallon type). Also I suspect cheap compressors might leak some oil to pressurized air? So to get 1000 liters of air to a small light weight bottle on high pressure for most of us is not a good idea. And Scuba tanks weight > 10kg and hold about 2000l of air at roughly 200bars.
For example small 1.5l tank with 200bar = 300l of air which equals one and half kites filled.
So to summarize, electric compressor sounds better option than air tank.
As I talked with liters (l) which is SI unit (Système International d'Unités). SI unit liters to US gallons, 1000l = 227 US dry measure gallons which is 264 US liquid gallons
SI unit for mass is kg - 5-6kg = roughly 11-12lbs.