If your rules don't specify then here's what I do. I play HOTT, DBN and DBA-HX so this works for me with all three rule sets.
Heavy Horse - Cuirassiers, Pistoliers, etc. 4 per base
Cavalry - Dragoons, Horse who charged in mass at the gallop 3 per base
Light Horse - Skirmishers, Hussars, Chasseurs, Cossacks, Carbineers
I also base all of my mounted figures on 40 x 30 MM bases
My two cents and hope it is helpful.
Terry