In 45 litres you could fit two =)
My personal rule of thumb is 3 UK gallons each of water, plus extra space for decor and substrate. Some would say 5 UK gallons, and some would day more, but I think 3 UK gallons gives you enough space to make the tank interesting for your betta and to give him enough room to swim and keep fit.
I prefer a long tank rather than a deep one, so they get the max horizontal swimming space.