Inside or Outside?
This is a huge factor and that is why it is the first to address where the location is. Piping inside amplifies the sound and even large churches and event centers one bagpiper fills it up. I have done multiple pipers and drummers indoors and I will only do that as an option with big building, not the local Elks hall. It's a courtesy to the musicians and the patrons and I normally want to be hired again and not be ran out of the building for making everyone deaf. Outside the sound dissipates greatly and that is when multiple musicians would be beneficial but not always necessary. I've played beach weddings, and they always have crazy wind going and I've never had anyone not hear me lol. A bonus to several bagpipers that play well together is the great effect of all that sound and the opportunity of harmonies in the music.
Drummer(s) as well?
Just like having several pipers, having drummer(s) can be a great effect to add to the event. The standard math I use is in most settings 1 piper and 1 drummer is better than 2 pipers (not always the case like funerals) and you can have just 1 drummer for up to 4 pipers in my opinion before it gets very lost in the mix. Then bigger and bigger you add more a bass drummer, more snare drummers and for the big events and flare tenor drummers. With all those musicians we are talking a big group of minimum of a dozen. There are many other things to do but I haven't had to book a full pipe band... yet.
Budget is always an issue when talking 1 piper or a full band and I'd like to share a few truthful secrets if you'd like to read. Booking pipers and drummer is not like going to Sam's Club and the more you get the better the deal. Every musician spends years working on the craft and thousands on their instruments (except drummers and spend hundreds on sticks), thousands and thousands of hours practicing and studying and subjecting family and friends to the horrors of listening to the learning of the instrument. I always do my best to be fair with cost but I need to make a living and eat once in a while.