The number of stored PFTs is only limited by the phone or tablet's local storage. And since each PFT takes very little space, we'd expect that hundreds of PFTs can be stored.
This will vary greatly by the device. For example, if you have an older phone and have taken lots of photos or installed many other apps over the years. That could reduce the # of PFTs that can be stored. Howver, just by deleting a picture or two, you can make room for tens of PFT tests.