There are uncounted hundreds, arranged in order of age.
From A Game of Thrones, chapter 66, Bran (all emphasis added):
Summer stalked out in the echoing gloom, then stopped, lifted his head, and sniffed the chill dead air. He bared his teeth and crept backward, eyes glowing golden in the light of the maester's torch. Even Osha, hard as old iron, seemed uncomfortable. "Grim folk, by the look of them," she said as she eyed the long row of granite Starks on their stone thrones.
"They were the Kings of Winter," Bran whispered. Somehow it felt wrong to talk too loudly in this place.
Osha smiled. "Winter's got no king. If you'd seen it, you'd know that, summer boy."
"They were the Kings in the North for thousands of years," Maester Luwin said, lifting the torch high so the light shone on the stone faces. Some were hairy and bearded, shaggy men fierce as the wolves that crouched by their feet. Others were shaved clean, their features gaunt and sharp-edged as the iron longswords across their laps. "Hard men for a hard time. Come." He strode briskly down the vault, past the procession of stone pillars and the endless carved figures. A tongue of flame trailed back from the upraised torch as he went.
The vault was cavernous, longer than Winterfell itself, and Jon had told him once that there were other levels underneath, vaults even deeper and darker where the older kings were buried. It would not do to lose the light. Summer refused to move from the steps, even when Osha followed the torch, Bran in her arms.
"Do you recall your history, Bran?" the maester said as they walked. "Tell Osha who they were and what they did, if you can."
He looked at the passing faces and the tales came back to him. The maester had told him the stories, and Old Nan had made them come alive. "That one is Jon Stark. When the sea raiders landed in the east, he drove them out and built the castle at White Harbor. His son was Rickard Stark, not my father's father but another Rickard, he took the Neck away from the Marsh King and married his daughter. Theon Stark's the real thin one with the long hair and the skinny beard. They called him the 'Hungry Wolf,' because he was always at war. That's a Brandon, the tall one with the dreamy face, he was Brandon the Shipwright, because he loved the sea. His tomb is empty. He tried to sail west across the Sunset Sea and was never seen again. His son was Brandon the Burner, because he put the torch to all his father's ships in grief. There's Rodrik Stark, who won Bear Island in a wrestling match and gave it to the Mormonts. And that's Torrhen Stark, the King Who Knelt. He was the last King in the North and the first Lord of Winterfell, after he yielded to Aegon the Conqueror. Oh, there, he's Cregan Stark. He fought with Prince Aemon once, and the Dragonknight said he'd never faced a finer swordsman." They were almost at the end now, and Bran felt a sadness creeping over him. "And there's my grandfather, Lord Rickard, who was beheaded by Mad King Aerys. His daughter Lyanna and his son Brandon are in the tombs beside him. Not me, another Brandon, my father's brother. They're not supposed to have statues, that's only for the lords and the kings, but my father loved them so much he had them done."
From this description we can see that:
starting from the stairwell, they see very old kings first, from hundreds or thousands of years ago
the kings and lords mentioned seem appear in chronological order
the most recently deceased are right at the end - this is confirmed by an earlier chapter from Ned's point of view:
Ned stopped at last and lifted the oil lantern. The crypt continued on into darkness ahead of them, but beyond this point the tombs were empty and unsealed; black holes waiting for their dead, waiting for him and his children. [...] There were three tombs, side by side. Lord Rickard Stark, Ned's father, had a long, stern face. [...] In two smaller sepulchres on either side were his children.
All of this taken together pretty much confirms that the statues and tombs are arranged in order of age. As for the numbers, we see in the long quote above that there are thousands of years' worth of kings, enough to fill more than the entire length of Winterfell on one level, with even more on lower levels. Several hundred statues would be a conservative estimate.