The achievement says "On a Whim" "Have a Sim complete 10 wants at a single social event."
Throwing a social event, like a dinner party, it becomes very difficult (if not impossible) to achieve this. The wants include things like "Get a promotion at work", "Work out at a gym", etc. These are not possible during a social event because they require you to leave the lot. I even tried liking a bunch of music so the first two or three are just listening to a specific music type, but even then, I run into these difficult ones. I even get ones for becoming friends with someone who is currently an enemy and not at the event. Hard to get even that one if that is the only one. Please make this possible to achieve again. It used to be you could swap out the whims for others, so this achievement was possible, but I've tried several times with no luck. It would be different if it was the whims of the household, but it seems to be 10 for one specific Sim.