Before armor they all have a pool of 20. With armor (from memory)...
Profession pool/regen:
Warrior 20/2
Ranger 25/3
Caster 30/4
You can get items with -energy regen that have big boosts to the energy pool, both a wand (+15?) and an artifact (+27?). A staff +15 max, an artifact +12 normally, a wand with a conditional +5 (like health over 50%).
Certain armors add additional energy - gladiators, druids, tats, scars. Others have -energy.
No limit, other than the items available and your willingness to obtain and use them.