The imposition is not so much the number of kids, but the duration of it...an entire weekend is a very long time (nothing short of abuse), for a single elderly person to manage one small child, let alone five.
This conflict will replay itself repeatedly going forward, every time she needs her monthly 'break'! So there is the potential here for more problems and more bad feelings. Best to put your foot down now...'rip off the bandage all at once' as they say. Decide on which and how many of the children you are able to manage together comfortably, and then choose which day and for how long. Going forward you need to set your own rules....rules that suit your life, not hers. She won't like it, but better she get over it sooner than later.
Does your DD have other close friends or relatives with as many kids? Perhaps they could swap an evening of child sitting so they could both get a break?
Sure....an afternoon or evening with her dear friends kids in tow (and her own) is no less a challenge, but a few hours is not an entire weekend, especially if both (Mom and Dad care sitters) plan a special activity for them. Two families would be rewarded with much coveted and perhaps more frequent breaks and at least one (justifiably tired) grandmother will be off the hook.