I'm not a massive HA fan, but 50 arrows sounds about right for them (a quote in something I recently read on the Mongols). They have a horse to carry the stuff, plus archery is their preferred fighting style (rather than a preliminary to the 'real' fighting), so it is natural that they should have more ammo.

Of course, balance will be an problem. I recently tried cutting cavalry regiment sizes by half to ease pathfinding issues, so I could easily justify doubling the ammo count of all missile cavalry. Full-sized HA regiments with twice as much ammo will be a nightmare to face.