The standard recommendation on this site is no more than 3% of your soaping oils weight (3% is roughly .5 oz PPO). That is below the therapeutic threshold for most essential oils.
That said, some should be used at much lower concentrations. Peppermint for one, and spice oils like clove or cinnamon because they can be irritating.
There are many, many threads on this topic that have been posted here within the last few months. Do a search and you will find them. Lindy and Shawnee (Alchemy & Ashes) are very knowledgeable about EOs. That said, there are good reference books that you might consult like those written by Valerie Wormwood and Julia Lawless. Martin Watt is great but unfortunately his books are very hard to find/expensive.