In The Mabinogion's tale of "Branwen uerch Llyr," There is a horrific battle which is said to have killed all of Ireland except for five pregnant women, and all of the army of Prydein (Welsh Britain), except for seven men and one woman:

Now, as much of the battle was over a Cauldron of Rebirth, I feel it necessary to bring up a poetic curiosity: In the 9th century poem "Prieddu Annwn" aka "The Spoils of Annwn," attributed to Taliesin, the refrain is "Save only seven, none returned from..." whatever name he was using in that stanza for the Otherworld. This is curious because in the poem, the quest is to steal a cauldron from the Underworld/Otherworld. But leading this quest isn't Bran, but King Arthur. The seven aren't specifically listed, but one can assume (though it takes one up to eight men and not seven, keep in mind King Arthur is perhapse doubling the role of Bran):

Now, I use the story of "Culhwch ac Olwen" because it specifically tells of a raid on Ireland (which stands in for the Otherworld) for a magic cauldron, very similar to the one in Taliesin's poem. And who is said to go on this voyage but Arthur, Taliesin, Manawyddan, Pryderi, Bedwyr, Gwalchmei, Cei, and Llwch Lleminawg, among others. It is likely that whoever wrote the "Prieddu Annwn" may have been thinking of both tales--"Branwen uerch Llyr" and "Culhwch ac Olwen" when he wrote the poem. (Obviously Taliesin didn't write it, because he died in the sixth or seventh century.)

In the grail romances, the seekers of the grail are usually: