I have several Mounted Bands in my collection - all of which I have made up from castings. I regularly choose between the following instruments - (in alphabetical order):
Bassoon, Bombardon, Clarinet, Cornet, Euphonium, Picallo, Sax, Tenor Horn, Trombone, Trumpet, Tuba. That's in addition to (usually) a Kettledrummer and Musical Director. There are some exceptions of course - see my latest Canadian Mounted Rifles with mounted pipers and three sorts of Drummers!(Note that British mounted bands never have a French Horn as one of the instruments).
It would be perfectly possible to use multiples of the above list though - as I sometimes do - and make up a band of over 20 quite easily. I usually also have an "Escort" with a band to get to at least that figure - as I think that a group of that number always looks better than a smaller one.
I have just commisioned a band of the 1st Lifeguards in review Order, from Yeomanary miniatures (as castings) which will have 27 members. I'm really looking forward to painting that - and hopefully, it will look excellent when completed sometime next year. Maybe we can compare notes with a Hiriart version when it's ready? jb