I'd be surprised if Casa Labra could seat 12-persons together. While this is a great place, historical, I don't think it can accommodate you. Plus, it'll likely be warm and you might like someplace outside, either in the shade or under an umbrella.
Take a look at the various terrazas-bars along the Calle de Las Navas de Tolosa-Calle de las Conchas (same street, just changes names) just north of the Calle Arenal between Puerta del Sol and Plaza de Opera (a.k.a. Plaza Isabel II). For exampel, "Casa Valiño" on Calle Trujillos, 9 (on the corner with the previous streets I just mentioned) or the "Restaurante Cervecería Doña Juana" on Calle Postigo de San Martín, 5 (on the corner with Calle de Las Navas de Tolosa). At these two places they serve good, pretty cheap(er) typical tapas/raciones with a great selection and they can surely push 3 or 4 tables together to accommodate 12 people. I had drinks and tapas at these two place just last week, but have been several times before and am always happy to find they're less frequented by tourists and more by Spaniards/locals. Spanish wait-staff, too. The above 2 places also have indoor tables, too, and could probably also push some tables together if it's too hot outside. Finally, they both have pedestrian-street terrazas on the north side of their bars/restaurants so you might well avoid the sun at 5-7pm or so.
Have fun!
Saludos, MadridMan
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