Minimum technically is three... But the more, the merrier!
They feel so much more secure, and are less likely to get startled or scared.
Though MALE guppies are colorful and some Bettas hate that... I have personally kept them together. The endlers always "flared" which is adorable. The guppies seemed to accept the Betta as a giant guppy
But this was an old boy who I raised myself, and he was relaxed with any fish.
You could do female guppies, who are less colorful. I have kept sororities or males with them without an issue. But that is only MY experience, I cannot speak for how it will go for you!
Certain Tetras are pretty common to keep with Bettas. Most of them are very relaxed, and would rather flight than fight!