I have been on NIle cruises with anything from nine to ninety-six passengers on board and, whether the cruise lasted three, seven or ten nights, there was ALWAYS a party night. However, as has been pointed out already, attendance is not compulsory - I either sat out on deck (you are unlikely to be the only one not partying) or went to bed early.
Note also that the Manager goes through the passports (or so I understand) , noting any forthcoming birthdays during the trip so that the Birthday Boy (or Girl) may be surprised after dinner by some of the crew processing around the dining room, playing musical instruments, before the presentation of the Cake. If any passengers will be celebrating an anniversary (for example) they may well request something similar on their special day. It is just a bit of harmless fun and does not take long - but it would be churlish to refuse a small slice of cake.