Shorebird,
10 as each reel is 10 bigger then the one before
It all depends upon which 525 you are comparing them to. The 535, 545 & 555 are made with the spool and spindle in one piece. Each one gets progressivley bigger with the numbers. For long range beach/light boat work with lines below 0.40mm (20lb) then the 525 is king. 20-30lb I'd go for the 535 amd so on.
My favourite is the 525Mag (slidy version) built like a Sherman tank with but with the Corvettes racing pedigree.
I hope this is of use - Led.