depends what type of fish you want, 20gallons is enough for most of the Target species of Puffer but you wont really see them that much as they mostly all hide apart from at feeding time.
this would include:
T Turgidus
T Cochinchinensis
T Abei
there is also:
C Salivator, which is supremely aggressive, mine were nastly little buggers (bit me on more than one occasion)
C Travancoricus, Dwarf Puffers, active but again nasty little buggers and can be very very nippy.
there are obviously loads of other fish types availbale, but these are smallish oddballs.