Going strictly by what we've seen in the shows Romulus has had very few colonies mentioned. I imagine that Romulus is seen as being the promised land due to it being the world chosen by their ancestors. It's possible that most of the other inhabited planets have a small Romulan presence, mostly military overseers using the indigenous population as slave labor.
It's possible, but it's not likely.
* The Romulans seem to be as expansionistic a culture as any other. To assume that Romulans didn't colonize new territories when they became available would be to have them behave in a way that no human imperialist power has ever behaved. The novelverse has further established that there are
multiple sizable Romulan colonies, the Praetor herself being from one (Glintara). Romulan colonies may plausibly not have been mentioned in the various series because non-Romulans didn't have access to the Romulan colonies; the Romulans are, after all, isolationists.
* It's very likely that the Romulans would take over inhabited worlds and establish themselves as ruling classes. That's what imperialist societies do. The novels even explicitly establish them as doing that with the Kevratans, while Terix II--a major Romulan world--also has its own indigenous population. The Romulans are almost certainly minorities on many worlds in their empire.
Given these are planets with populations possibly amounting to the billions, this is still a sizable number. For comparison, in South Africa immediately after apartheid
of the forty-odd million South Africans only five million were white. South African whites still are more numerous than, say, New Zealanders or Uruguayans, and they controlled a technologically and economically sophisticated state. Apartheid ended in South Africa as peacefully as it did only because whites were convinced to do so. I really don't see Romulans on these Romulan-minority worlds as being nicer.
Also, the Hobus supernova wouldn't have destroyed just Romulus but everything in between as well as in other directions. It's an expanding sphere. Spock may have used the red matter to stop it from entering Federation space. The rest of the RSE may be essentially depopulated.
The novelverse draws upon Star Trek: Star Charts
, which shows the Romulan Star Empire to be a pretty large ellipse of space that at points comes quite close to the Federation core, the novels further establishing the existence of large Romulan population on worlds fairly distant from Romulus--Achernar, Devoras, and Rator all come to mind. If the Hobus supernova really was so big as to annihilate all these Romulan worlds, then the Federation core worlds would also be destroyed.
The Wormhole wrote:
All that considered, I'm sure there are still other Romulans around, including a colony world or two. Certainly the days of the Romulans being one of the quadrant's biggest political entities are over, but the Romulan species is still around, even if there are very few left.
There's still going to be large numbers of Romulans around. Even if the Hobus supernova destroys some older Romulan colony worlds, the Romulans have spread out sufficiently that I can't see Romulan civilization as being doomed by the destruction of the homeworld.
(Will the destruction of the Romulans encourage the survivors to consider new possibilities? Sure. They're just not going to be driven into extinction, that's all.)