This is just my thoughts on where there might be allies lurking around in the Galaxy, who will actually come to the aid of the splinters of the Resistance.
I'm thinking that the New Republic took a massive blow when several of their worlds blew up in The Force Awakens. But from the Aftermath books and other sources, we might conclude that there are still many worlds that might want to be part of a wave of resistance in order to fight the First Order. They might be afraid, obviously, but they would probably know that the Starkiller Base was also destroyed after that attack. There might still be a lot of troops and ships left as well, who weren't circling or docked on the few worlds that were destroyed by Starkiller Base. I think they might be willing to take the fight to the First Order as the FO is now reeling from the destruction of both the super dreadnought, the Starkiller Base and also the death of Supreme Leader Snoke.
So there might be many who are looking for revenge. And survival.
I'm also thinking that Kashyyyk would still harbor many Wookiees who remember the slaughter and slavery of the Empire and would still be in debt to the remnants of the New Republic. AKA the now resistance/rebels, while also wanting to fight the First Order as they don't want to end up in the same way once again.
I'm also thinking there might still be a few people from the old Ghost crew still alive. We know Sabine Wren is probably still alive, as well as perhaps Ahsoka Tano (who would be an old woman, but still "a grey jedi" by now. Ezra Bridger would be as close to a jedi one could imagine, if still alive, and Hera Syndulla and her (perhaps even force sensitive) son with the now dead Kanan Jarrus would probably be around still. And that would mean there would be connections with the twilek people as well. Also, where is Wedge Antilles? Not dead, is he? So he should be around still? Somewhere? Perhaps returning from a mission somewhere? Or just living elsewhere and might return from pasture, like the old squad he created according to the Aftermath books.
Not sure how many bounty hunters and smugglers, mandalorians (since they might follow Sabine Wren nowadays, after all) might be interested in joining the resistance. And we know nothing about old Lando, except Bespin is still around (in one way or another, according to the Aftermath books). He might still be alive, though, and would probably want to avenge the death of Han Solo. Maz Kanata might not be too happy about her old castle being blown to bits. And she has quite the leverage in some very influential, however "scruffy looking" circles. What might she be up to?
By the way, I am quite certain that even though Luke Skywalker is now a force ghost, he will be a very powerful one. And we have NOT seen the last of him, for sure.
And what about Thrawn? Is he really that happy about the First Order, if he is still alive at this time? And would he and the Chiss feel they are truly safe from Kylo as the new Supreme Leader? Or will they rise up and decide they are safer with the First Order gone?
So I think that even though no one knows anything about what kinds of allies Leia is referencing at the end of The Last Jedi, I think there are many who might turn out to be potential allies and very well much needed reinforcements to the very few people who made it out in the end.