Every band needs a drummer and bass player to set the feel of every song. If you desire any rock, whether it be from the 50’s to the 90’s, a guitar is a must. An electronic keyboard provides the most variety of sound of any instrument, because it can emulate a piano, organ, synthesizer, and string sounds. these four instruments, plus the two or three vocalists should be able to cover many musical styles. Often the players are also vocalists, so you might be able to get by with a 4-piece band. Disco, Motown, soul, funk, and Sinatra-style standards sound best with live horns (start with sax, then trumpet, then 2nd sax or trombone). A 6-7 piece band can provide a full sound and a good variety, as long as a few of the musicians also sing. At the other end of the spectrum, there are talented 15-piece bands with multiple vocalists, percussionists, and even violinists. The only drawback is that you pay for musicians that aren’t really needed on many of the songs.