No, you don't have to entertain. If you don't, expect that you will have 12 girls entertaining themselves. That likely means that different groups of girls will help themselves to all the toys, games, play equipment, dress up costumes, anything that looks like it is fun and interesting. They will run around the house and make messes. It may be more likely that there is some exclusionary play or arguments between some of the children if they want to play with the same toys. You might have more trouble rounding them all up when it is time to move on to food and cake and presents because there's no natural finishing point to their play. That's all fine, if you don't mind.
For parties at home, I liked having an activity like a craft or a game. It gives the party a little focus, prevents the kids from pulling out all of our toys from the cupboards and shelves, and provides a loose schedule for the party. Once the activity is finished, you can move on to cake and presents and then the party is over.
Games have been things like an "Amazing Race" scavenger hunt (involved some planning before the party), pick up hockey game (easiest party - involved no planning since everyone brought their own sticks), spa party (this was part of a sleepover, we did manicures, pedicures and facials with the girls) and dress up party (also easy since we had lots of costumes and hand-me-downs). Crafts have been painting a terra cotta pot for a flowering plant, decorating photo frames, and making a ceramic plate.
We've never done the traditional party games - pin-the-tail-on-the-donkey etc. I preferred one major activity that needed a maximum of an hour to complete and then it was over. Then a half-hour for cake. The whole party is over in 2 hours, maybe 3 at most. That was more than enough excitement for all of us.
YMMV.
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