20 people. In Disney. In. July. Just shoot me.
Now, I've been in August many times - yes it's hot but not as bad as July. And the crowds are very low then (our kids go back to school after Labor Day, but many schools are already back).
I've also been ON Christmas, and after Christmas twice. NYE yes done that too. But - on site, and been many times before so we enjoyed the resort a lot more, did other things, like Illumiinations Cruise (on Breathless) on NYE (concierge booked it - I had no idea what it was, wow). Yes, Christmas week is nuts. But, we were prepared and went to the parks for a couple of hours, then back to the resort (concierge level, so heck it was good).
July is just super hot. And you're taking that many? I would just die. But that's me - Disney is very hard with more than one family (I was a travel guide before I got married for my folks plus 7 business friends of my dad's from Japan - I kept them going all day and it was fun but a LOT of work, and these were the days prior to touring plans, I had to do all my research in books!). I would do it again in a heatbeat - if I were still in my early 20's and didn't have obligations! No way now.
If I HAD to go with family with that many (and I wouldn't - I'd do anything else but Disney, it's just too much money and too many people wanting too many different things) I would pick a SLOW time of the year (and not the worst in terms of heat - July is the worst month for heat). I'd do something like the Tyler Place instead.