In my experience you usually get a few more than you paid for, especially if you get on fine with the sonographer.
I've paid £2/photo Queen Elizabeth in Woolwich, £4 (I think, could be totally wrong) at Kings College, and more recently £5/1, £10/3 at St Albans City Hospital. I was there today, paid the £10 and got four.
It does annoy me too when you get several pretty much identical photos, and often, as AhoyAhoy says, the same one but zoomed in. Badly. I mean today I got four photos but they're all pretty much identical, and even more annoyingly, they're not very clear. Seconds before she had a great clear shot but didn't press the button on that, instead she moved the thingy, it went messy, so she printed that. And another three subtle variations on it. I mean, huh? Oh well, at least I got a bonus one.
Add to the fact the appointment was half an hour late due to admin cockups (nobody had registered I was there despite me talking to the receptionists) and I had to pay an extra £2 for car parking on top the £4 I had paid already, I'm feeling ripped off..