In what precise context did you hear this? "Band" is not a precisely determined or regulated term that I'm aware of.

You might be interested in this thread about medieval military organization. This other series of answers by by u/MI13 might also interest you.

Of course, neither of those threads are definitive.

The sentence i heard it in i think was "i have a band of knights that can assist you in battle" it was from a movie i saw recently on television