Depends on the size of the tank (Triumph or Harley Fat Bobs?), and the type of paint, sometimes the color of what's underneath.
Base/Clear, count on a pint of color, (you'll probably use less, but better to have a bit too much, than run out!)and maybe 2 pints of clear, if you have an HVLP gun, they waste less. Solid color enamel? Probably half a quart or so, extra coats needed for sanding and buffing. Lacquer? I'd get a full quart, you need to put on many more coats than enamel, and still need to sand and buff it.
You can save a bit of color, if you are painting over a similarly colored primer or sealer. I've used only an ounce or so of black, putting it over a black primer, and then clearcoating it, so none it gets sanded off.
Also, some colors are more transparent and need more coats to cover well. Sometimes twice as much material. Pearls, red metallic, greens, golds. To save paint,, again, use a similarly tinted color sealer/primer.
Cheaper versions of paints (Nason vs. regular Dupont) usually need more material to get full color, as well.