Both 5 blade. The px5 is 15 in diam. Maximas 15.5. Will they turn similar RPMs? About 5500 rpm range

You have a 38 foot boat. I think 15 inch Diameter is too small of a prop. I can't answer your question about RPM, but if you are switching from 4 to 5 blades, you need to go down 2 pitches to keep the same RPM range.

15" is not too small. All the B1 below 34" are 15" in diameter. If both props are stock, it can be a bit difficult, simply because of the variables. One set might spin more than the other. It also depends on the boat. One might create more slip than the other. If you only look at the props, it could be broken down like this. Both props exactly 32" as a starting point. The Maximus has 30 degrees of rake, which is quite a lot and rake creates drag. I don't recall what the rake of a P5X is, but I'm pretty sure it's somewhat less. Then you have the 15.5/8" diameter compared to 15". In many cases the higher rake and larger diameter provides lower slip. By these factors the P5X will spin faster, but the particular boat in question is part of it too. There is no straight forward answer really, other than testing.

Stock versus stock the Maximus will turn the lower rpm.

Brett