Hi Lily! Thanks for bringing your question to planDisney.
I happen to have some recent experience dining with a large group at Disneyland Resort. It is doable! However, in most cases, you'll need to book multiple smaller group
dining reservations within about 15 minutes of each other at the same location to stay together. How big a party a restaurant can accommodate depends on the location. For example, you could book two parties of 10 at
River Belle Terrace in Frontierland at Disneyland Park. If you break up into three or four smaller groups, however, you'll find lots of dining options available early within the 60-day dining reservation window. When you arrive for your reservation, you could always ask the Cast Member at the podium if it would be possible to be seated as close together as is feasible for them to accommodate. I might even check in at the podium earlier in the day, and there's a chance the Cast Member could make a note of your request to be seated together.
An exception to this multiple reservation approach would be
Goofy's Kitchen at Disneyland Hotel, which allows up to 20 in a party on one dining reservation. Goofy's Kitchen is a fantastic Character Dining Experience that offers both breakfast and dinner dining options. The vast variety of tasty options appeals to both young and old, plus, you get to meet some of your favorite Characters! I'd highly recommend it, but book early. It's popular and will fill up, especially with a large group.
For Quick Service options, I'd recommend
Rancho del Zocalo Restaurante in Frontierland at Disneyland Park, since it has a large seating area and has some of my favorite Counter Service food in the parks. At Disney California Adventure Park, try
Boardwalk Pizza & Pasta in Paradise Gardens Park, which also has plenty of seating.
Visiting with a large group means even more fun with careful planning. I hope you have the best time!
Elizabeth