How many Bristlenose plecos?

One.

I wouldn't get more than one unless you can get a confirmed pair. As plecos get older, they can get awfully territorial with each other.

Also- you shouldn't need more than one. And make sure you feed it fresh veggies regularly, don't just leave it to scavenge.

What sort of veggies?

Just about anything. Lettuce leaves, zucchini, cucumber, squash halves, or carrots are what I usually use.

If you boil the veggies first they'll sink