They don't take reservations on a normal day, not sure about holidays

I don't see it on the menu. https://foursquare.com/v/hacienda-colorado/503fbf3ae4b074ec0865f320/menu

Hacienda does offer space for corporate events. Looking at the catering prices you can see they offer options to set prices per order. They can provide an estimate on how many people it will feed if you inquire directly with them.

No, there is no Sunday brunch.