My theory on the regis, is that there are more than one of each, because they have been seen on trainers at battle tower, and from what little anime I have seen, they appear to simply be a species of that guards things, rather than a one-of-a-kind sort of pokemon, like mewtwo, or the legendary birds. There are also references of them being "a regirock" not "regirock" as if there are are more than one of it in existence.

I was wondering if Im correct.

EDIT: Also, since there are regis found in hoen, and in sinnoh, does that mean they could be found somewhere, hidden, in or around jhoto or kanto? Another thing is regigigas, are there multiples of it too?

Hmm, that's a pretty good theory.

As far as the games go, there are at least two sets of legendary golems. Two Regigigas reside in Sinnoh (the event, and the one inside Snowpoint Temple) and the rest of the Regis are in hidden points in Sinnoh and Hoenn. NPC's such as Frontier Brains who own legendary PokeMon should not be taken seriously story-wise as they are usually thrown in to be difficult. Also, it has been suggested by the Braille writing found in the games that the golems were PokeMon created by humans and Regigigas.

This also brings me to another theory I'd like to address, the legendary birds. Some people believe that there are more than one of the birds. In the anime, this has been confirmed, somewhat. But in the games, the birds that roam Sinnoh are the EXACT SAME ONES that Red encountered during his RBGY run in Kanto. It is speculated that he is why the birds and Mewtwo are not in their places when you reach Kanto during GSC. The birds are said to have been disturbed from their nests by either Red, Blue, or (less commonly) Team Rocket. So in the games, Zapdos, Articuno, Moltres, Lugia, and Ho-oh are the last of their kind.

Yes , Its True There Are Multiples Here Are Were I Recall Seeing Them:
-Brandons Team (Game+Anime) Regi Trio
-Pokemon: Lucario And The Mystery Of Mew (Anime Movie) Regi Trio
-Pokemon: Giratina And The Sky Warior (Anime Movie) Regigigas
-Snow Point City Temple (Anime: Regigigas Ressurection! J Again!) Regigigas and Regi Trio

And thats what i can remember so far, if i remember any more i will list.

They were real, the anti bodies were a bright orange colour in shapes of pokemon, the regi's were real they protected it to stop intruders from entering, the anti bodies attack when ash and co. got inside the tree regirock attacked them before they reached it.