For those interested, most of the larger shows in Vegas (generally termed "Production Shows") use live musicians. Some of those shows include the Cirque shows (Mystere, O, KA, Zumanity <LOVE, based on the music of The Beatles, does not use live musicians, but, rather, uses master recordings from The Beatles>), Mama Mia, Phantom, The Producers, Spamalot, Avenue Q (now closed), We Will Rock You (now closed), Blue Man Group, Le Reve, A New Day (Celine), etc.
Bands/Orchestras range in size, with the largest probably being the orchestra at The Producers (approx. 25 players). Many of the shows are Union. However,
Nevada is a 'right-to-work' state.
Enjoy the music,
Aaron