How much ml can you put in the pyriform fossa?

It depends really on the experience of your injector but be mindful they should have years of experience as the nasiolabial fold and pyriform fossa is one of the more "dangerous" areas to inject because of the anatomy of the way the angular artery travels. I personally like to use a 25G cannula to inject that area to reduce greatly the risk of getting an occlusion. so 1mm-1.5mm i would suggest.

When considering filler it is important to select an experienced injector. Before treatment you should discuss your concerns and goals for treatment. Your injector will make a recommendation based on an evaluation and your goals, including the amount of filler needed to address them. This will vary by patient. I've included a video I hope you find helpful.

Piriform injections can be very powerful to help treat deep nasolabial folds! I typically inject 0.5cc-1cc per side in my patients to soften the depression.