It does depend on the breed. The swiss breeds (British Toggenburg, British Saanen, British Alpine's etc) tend to have 2 or 3 on average, but can regularly have single's, and occasionally 4 or even more occasionally 5.
Anglo-Nubian tend to have more multiple births, so I would actually say triplets is pretty normal, with 4 or 5 happening quite often as well.
I would say that a younger animal tends to have less- certainly the first time they kid a single or twins is more the norm.
This year we got quads from a 4 year old British Toggenburg type, twins from 2 AN first kidders, another AN first kidder had 1 kid, plus a bag of bones ( a partially reabsorbed kid). A BT second kidder had triplets, a bT first kidder had twins.
Oh and my 4 year old AN kidded in May- looked like she had at least 4 kids in her (had 4 the previous year)- and she only had twins!
They do produce more milk if they have a multiple birth- but there is ultimately a payoff, TOO many kids might put too much of a strain on them and then result in them not milking as well. Our BT type did this- having 4 big kids just took so much out of her, she didn't hit the 7litres a day production she did as a first kidder. She still gave 5.5litres, but her milk dropped off quicker as well.
Beth