The highest natural riding requirement is 4 (Warhorse and Charger). The Spirited Stubborn modifier adds +1, while Champion adds +2. Spirited horses are somewhat obtainable but Champions are very rare and the odds are that you won't find a Champion Warhorse or Charger any time soon. Riding skill beyond level 6 presumably still increases skill and manoeuvrability to some degree (I have never bothered to max it out) but no horse can need a skill higher than 6 outside of mods.