Recruited from the finest troopers of the alae, the regiment's original core component were the crack Batavi horsemen. The regiment was expanded to 2,000 horse in the early 3rd century by the emperor Septimus Severus. It appears that after some campaigns, detachments of singulares were left behind in the provinces, to form the core of new regular alae, which retained the prestigious singulares title and elite reputation e.g. Ala I Flavia singularium stationed in Raetia in the mid 2nd century.
Historical information courtesty of:
Goldsworthy, Adrian (2003) The Complete Roman Army.