what was the unit size?
Each Polybian legion contained a cavalry contingent of 300 horse, and the cavalry contingent was divided into 10 "turmae" of 30 men each
Roman cavalry
Each Polybian legion contained a cavalry contingent of 300 horse, which does not appear to have been officered by an overall commander. The cavalry contingent was divided into 10 "turmae" (squadrons) of 30 men each. 
what is the emesa helmet? A Roman cavalry helmet can you please describe it? It consists of an iron head piece and face mask, the latter of which is covered in a sheet of silver and presents the individualised portrait of a face, likely its owner. when was it discovered? August 1936 what is its function? The Emesa helmet is highly functional, and was likely made for both parades and battle. where was it discovered? The modern-day city of Homs is it on display anywhere? Yes, the National Museum of Damascus. has it been restored? Yes,  the Emesa helmet underwent several unsuccessful restorations, eventually it was taken to the British Museum, where a final restoration was finished what was the helmet made out of? Iron what other equipment did the cavalry you mentioned before use? A Corinthian helmet, mail armour, saddle how was the recruitment done? Large numbers of young men from the First Class of commoners were regularly volunteering for the service, which was considered more glamorous than the infantry.
wiki:20550562
Roman cavalry [SEP] Unit size and structure
Each Polybian legion contained a cavalry contingent of 300 horse, which does not appear to have been officered by an overall commander. The cavalry contingent was divided into 10 "turmae" (squadrons) of 30 men each. The squadron members would elect as their officers three "decuriones" ("leaders of ten men"), of whom the first to be chosen would act as the squadron's leader and the other two as his deputies. From the available evidence, the cavalry of a Polybian legion (and presumably confederate cavalry also) was armoured and specialised in the shock charge.