Like all of the Legiones Astartes the Thousand Sons were initially made up of Terran marines. In the aftermath of Magnus' deal with Tzeentch to save their lives, only around a thousand of them were left. For the rest of the Crusade, the Thousand Sons recruited from Prospero, a planet of only limited population (although many of them were psychically active). As a result the Thousand Sons were never a large legion.

How the Thousand Sons currently maintain their numbers is unknown, although they have been observed carrying out an apparent resurrection ritual at least once. On this occasion the spirits of deceased Thousand Sons were summoned from the warp into mortal bodies (whether living or dead making no particular difference), bodies which then transformed themselves into reborn Thousand Sons.

At least one Imperial commentator on the subject believes that the Thousand Sons have a cult network that provides potential recruits for the legion.