Actually, what you have is two mutually contradictory statements, both from RbS. A fleet is said to have 8-10 squadrons of 6-8 ships. You're not going to get 28,000 ships from 320 fleets that averages 63 ships. OTOH, the statement dovetails quite nicely with the 20,000 ship figure.
Those 8-10 squadrons are only crurons and batrons. each has a desron or two as well.
So we get a maximum of 320*8*10 = 25600 maximum ships of the line, and a minimum of 320*6*8= 15360 minimum.
When we get into the MT era, we have spinal-equipped desrons - which probably accounts for the discrepancy of around 8K ships... and indicates about 1/3 to 1/2 of the desrons are big enough destroyers to be ships of the line.