depends what size tank, male/female. If its a large tank about 29gallons then you could keep one male betta with 4 or 5 males guppies. And you could keep a a couple female bettas with a couple male/femal guppies in a smaller tank
I do believe it depends on the betta . . . some males are more aggressive than others . . and some guppie males are very pretty . . . he might get jealous and rip their fins . . . but for the most part try it out and watch the closely for a while. . .i on the other hand will never do that because i sell my guppies and want their tails to look good : ) good luck
you have to have at least 2 or 3 more to spread out there agression ( like cichlids) Plus a decent amout of space (29 gal and up) and a few plants /rocks /etc