"The North remembers" has been said a couple times in Game of Thrones, in reference to the death of Eddard Stark or to the pillaging of the northern houses and the North itself. Hell, it's even been the name of an episode way back in Season 2, and it helped a distraught Sansa regain some hope when she heard it in Winterfell.
Problem is, in this week's episode, Sansa and Jon were faced with the stark reality that the North may not remember so much after all. In their campaigning of the North trying to rally support, they paid a visit to two important houses and sent ravens to a couple more, but came up quite short of the army they'd expected to gather. Some don't believe they can overthrow the Boltons, others think the Starks are dead and, I imagine, most simply fear the Boltons more than they're willing to pledge to the remaining Starks.
But why should the Northern Houses join with Sansa and her army?
The Starks had been the Lords of Winterfell since the First Men. They ruled as Kings of the North up until the Targaryen Dinasty came about and Torrhen bent the knee to Aegon the Conqueror. Hence, the Starks became Wardens of the North and Lords Paramount of the North, with every other northern house answering to them, as representatives of the king.
None of this is no more.
The last Lord (Stark) of Winterfell, Eddard Stark, is long dead and the Boltons have taken his place both as Wardens, as well as Lords Paramounts of the North. So, in theory, every house in the "grey waste Ned Stark called home" now answers to the Boltons, or better yet, to the former bastard and patricide Ramsay Bolton. The catch here is that, because the Boltons played a huge part in unseating the Starks, the northern houses are somewhat divided in their support. The "North remembers", you see.
Team Bolton v Team Stark
The Boltons and Starks have had a bittersweet relation since the beginning of time. Both houses are descendants of the First Men and, after centuries of fighting and flaying and more fighting, the Boltons finally submitted to the Starks. Clearly, that animosity persisted and lives on - though I'm willing to bet the clock is ticking on it.
According to wikipedia, in George R. R. Martin's books there are 69 houses in the North. Out of those, the original Stark bannermen - the ones who swore their obediency - were Houses Reed, Glover, Mormont, Umber, Manderly, Flint, Wull, Norrey and Liddle. The Boltons answered both Eddard's and Robb's calls to the banners, the latter dying at the hands of Roose himself.
In the books at this point, most of the Stark bannermen have allied with Stannis - who is still very much alive - against the Boltons. The show has given us a new story, making it a bit harder to see where fealty lies. This episode gave us the Glovers and the Mormonts, the latter pledging 62 men, while the former declined any help saying the "Starks are dead". Ouch! Sansa was right there, dude!
Houses Karstark and Umber are pledged to the Boltons, alongside Glover. The Karstarks had their Lord beheaded by 'King in the North' Robb Stark, and the Glovers had their castle taken by the Ironborn while Robb did nothing. The Umbers have recently changed sides - or so it seems - by presenting Ramsay Bolton with Rickon Stark, who they'd been hiding so far.
That leaves Manderly, whom Jon mentioned they'd sent ravens to, and the ones that have joined their crusade: Mormont, Hornwood - their banners can be seen in the Red Band trailer - and Mazin. Sansa talks of sending word to House Cerwin and, even though Jon cut her off, I'm guessing that message she was writing will be flying to the Cerwins fast enough.
So will the Battle of the Bastards really happen?
It all points to it, whether Sansa and her army is big enough for it or not. Let's keep in mind what Jon said about the smaller houses, how they rival the noble ones if they stand together. There's also the matter of the Riverlands and the Tullys, who are Sansa's family through her mother and, theoretically, may come to her aid. I say theoretically, because as we've seen this episode, they're currently under siege. So, unless they're able to break it in time, Blackfish Tully may not join Sansa's army after all.
Last, but definitely not least, there's the Vale, under Littlefinger's influence. The Knights of the Vale were mobilized and, according to Petyr Baelish, are stationed in Moat Cailin just waiting for his command to join forces. Things may not be looking to bright at the moment - 2000 wildlings, 200 Hornwoods, 143 Mazins, 62 Mormonts and a giant - but Jon and Sansa still have houses to look for support. The battle will happen and the show is definitely closing in on it.