Tolkien does not tell us the size of the population of Númenor, or how many of them were faithful. He does, at least tell us that the faithful were in the minority.
On the one hand was the greater party, and they were called the King’s Men, and they grew proud and were estranged from the Eldar and the Valar. And on the other hand was the lesser party, and they were called the Elendili, the Elf-friends;
The Silmarillion: Akallabêth
Page 274 (Houghton Mifflin Harcourt; 2012 Kindle Edition)
We know that the faithful lived in the Andustar section of Númenór and its main city: Andúnië. From the map of Númenor in Karen Wynn Fonstad's Atlas of Middle-earth, we can estimate that Andustar less than a sixth of the area of Númenor.
It probably had much less than a sixth of the population as it didn't include either of the two largest cities (Armenelos and Rómenna) and only one of the second-tier cities (Andúnië and Eldalondë).
The population was concentrated in Armenelos and Rómenna, with large settlements also at Andúnië and Eldalondë in the west.
The Atlas of Middle-earth (Karen Wynn Fonstad): Númenor
We also know that the population of Andúnië was smaller at the end of the Second Age than it was earlier.
Originally, the port of Andúnië was the largest city, site of the pleasant seaside estate of Elendil. Eldalondë was also sizable, for the populace gathered there to receive the Eldar, who journeyed from Eressëa. As distrust of the Elves grew, however, the self-centered Númenóreans gradually shifted to Armenelos the Golden. Those folk faithful to the Valar and the Eldar remained in and around Andúnië until Ar-Gimilzor forced them to remove to Rómenna in about 2950. Thereafter the western havens were less important until they became the site of the embarkation of the Great Armament.
The Atlas of Middle-earth (Karen Wynn Fonstad): Númenór
From this, we can estimate that the faithful amounted to less than 10% of the population of Númenor, and probably much less.
After his father sails west to petition the Valar for forgiveness. Elendil embarks what appears to be all the faithful that still remain in Andúnië onto ships.
But Elendil did all that his father had bidden, and his ships lay off the east coast of the land; and the Faithful put aboard their wives and their children, and their heirlooms, and great store of goods.
The Silmarillion: Akallabêth
Page 283 (Houghton Mifflin Harcourt; 2012 Kindle Edition)
We are told in many places that there were nine ships that carried the faithful from Númenor back to Middle-earth.
Nine ships there were: four for Elendil, and for Isildur three, and for Anárion two; and they fled before the black gale out of the twilight of doom into the darkness of the world.
The Silmarillion: Akallabêth
Page 290 (Houghton Mifflin Harcourt; 2012 Kindle Edition)
I can find no description of the size of these ships or how many people they could hold. However, I doubt they could hold more than a few hundred people, which means that only a few thousand people escaped from the downfall of Númenor.
