None if you are really going to use it for sowing seeds, though this isn't the best time to be doing that.

If you are starting from scratch with plain peat, I would mix in 1 part perlite for every 4 parts peat. If using coconut fiber, about 1 part perlite for every 5 parts coconut. If you are making a John Innes mix, probably none, as MG says.

Half and half works for me.