Most 7 day or longer cruises have a couple of times set aside with the Celebrity orchestra for "Real" dancing. You have to watch the daily program for the specific times....
Other times every day before dinner there is dancing in one lounge or other. Usually is called the "Rendezvous Lounge"
There will also be various dancing in various lounges throughout the evening and later into the night at the "Disco"
We have been on the Millennium for 6 cruises and found that the Rendez-vous lounge usually has ballroom type music - mainly fox trots, rumbas, swing, waltzes, cha chas. However the dance floor is small (there are only two dance floors on the M-class ships - the Rendez-vous which is outside the dining room and one in the lounge - the Cosmos on Millennium - which may have the party band play sometime during the evening and then is the DJ and disco). The Rendez-vous is very crowded between the two dinner seatings - so much so that you can't really do ballroom as most people just dance in place. But about the last couple of hours after late seating, the dance floor empties out and serious dancers take the floor.
I've never found that to be the case with overcrowded dance floors...but then again, I'll dance anywhere, in the aisles works for me.
I think you'll be fine with finding plenty of room and places to dance. As I sit at the champagne bar which overlooks Rendezous, I've never seen it "overly crowded". I think it also depends on how long your cruise is, what your destination is, etc. Some cruises are predisposed to retiring for the night early, others not so much.