Gonzo and others are right, which is why it's really annoying when you get "blah blah blah, passing x thousand feet for x thousand feet on a y departure
with ident
Trouble is I'm only allowed to count you identified if I ask for the squawk ident, you acknowledge it, and then I see it on the radar at the aircraft I expect you to be. SO if you ident without asking, I haven't asked, and so I can't count it.
As for the number of planes we work at once - regularly 12 - 15 in the London TMA, occasionally more than that.
The chances of more than one identing are less likely if we ask for it, but it can happen, and remember that whilst squawk ident is most frequently asked for after departure from the major airfields, it can also be used for other traffic where we might not be as sure which is who.....
In the good old days getting you identing also used to set off the tracking system and ensure that your flight plan was activated. Is that still the case anyone?
Hope that helps.
Cheers,
Northerner
"Keep smiling, it makes people wonder what you're up to..."