I do own a Horse Ranch community building but how do you keep your horses there and does that allow you have more horses or is your limit still capped? My main problem is I simply can't part with any of my horses! My original intention was to 'rescue' wild horses by adopting and selling them. I got the adoption part down pat. The selling part? Not so much.
Owning community lots won't change your Household maximum Sims limit.
I think if you control the horses and just tell them to eat/sleep/train at the Equestrian Center they'll do so even without owning the lot. I adopted a Unicorn that autonomously preferred to hang out at the Equestrian Center all day and night.
I'm also role-playing a similar rescue ranch: "Wild Mustang Rescue and Troubled Teens Ranch", where I have a trained Horseman who adopts horses from the wild or the shelter, and adopts children, and the children have to take care of the horses after school. I find it easier to part with the horses by moving them out to a new house with one of the children I have raised.
My role-play ranch involves a rule that a child who ages up well to Young Adult gets to take with him whichever horse he has become Best Friends with. With aging off you have plenty of time to sell a few horses to level up the horseman career if you're going for that, and still let your sim children befriend one before they move out into the world.