You can stack as many bonuses as you want but from what I have experienced they stack additively and not multiplicatively, so the relative increase will become smaller for every added bonus.
But of course this was before Exploration was removed and before Training was changed to XP from kills, so I don't know if the 85% cap mentioned here still holds.ZOS_JessicaFolsom wrote: »Hi everyone,
How XP Buff Stacking Works in ESO
In ESO, all XP buffs stack with each other, with the exception of Psijic Ambrosia and the Crown Experience Scroll. Not counting the Jubilee Cake, these are the different types of XP buffs currently in the game.It is currently possible to stack XP buffs for a total possible increase of 85% for most types of content. There is normally a cap in place that prevents any event (a quest completion, a monster kill, etc.) from granting more than 10% of your total level value.
- Alliance War Keep buff – 5% XP buff (applies only to monster and player-character kills)
- Crown Experience Scroll – 50% XP buff (does not stack with Psijic Ambrosia)
- ESO Plus – 10% XP buff
- Grouping buff – 10% XP buff
- Psijic Ambrosia – 50% XP buff (does not stack with Crown Experience Scroll)
- Rings of Mara buff – 10% XP buff
- Exploration Trait XP buff – Varies depending on how many armor pieces have the Trait (applies only to exploration XP)