I've been rereading Spock's Wold recently and there's a passage that talks about how very few Vulcans have ever left their planet, let alone their system. The number quoted was around 5% as opposed to about an average of 40% for other races. Romulans are basically Vulcans under the skin.
And if the Romulan population is anywhere near the current population of Earth, that 5% would equate to 300 million people. If only 1% of the population was off world when the event happened, that would still be 60 million people. I think that would be more than enough for the Romulan people to survive.
Much like Spock in the new timeline, perhaps Saavik takes on the cause of finding the Romulans a new home and uniting those that are left.