I'd say it depends how big a band yoollu are.
If you're a small, pub playing band, theres no point charging back on stage and play the second half of your set as an "encore." It'd be like a Jazz player stage-diving and doing the whole hardcore neck-whiplash bollocks.
If you're a little bigger, then yeah, I'd say one encore is okay. If you're playing large venues, then yeah, 2, 3 encores, it's all good for the fans.
But I remember when I went to see Kasabian, they did like 8 encores or some high-end even number of them, and by the end of it, I was just like, "so yeah, you were supposed to be off stage 30 minutes ago, correct?"
Ah drunken band-members. *wipes tear.*