It's an equilibrium so it's a proportion. At a constant FC level, more CYA will have more FC bind to it but never all of it. Twice as much CYA will have a little more FC bound to it leaving about half as much active chlorine. Every doubling of CYA cuts the active chlorine in half. It never gets to zero. Of course, once the FC/CYA ratio gets below certain amounts algae can grow faster than chlorine can kill it. This is the basis for the

Chlorine / CYA Chart.

It sounds like you are thinking that more CYA linearly attaches to more chlorine, but that's not how chemical equilibrium works. It's ratios and proportion. Since most of the chlorine is bound to CYA, that amount doesn't change very much in absolute terms, but the amount that is left as active chlorine changes proportionately. As described in more technical detail in

this post, the relationship is essentially the following:

[CYA] * [HOCl] / [Cl-CYA] = constant

so solving for [HOCl] which is the active chlorine concentration we have

[HOCl] = constant * [Cl-CYA] / [CYA]

Since most of the chlorine is bound to CYA, [Cl-CYA] is approximately FC so

[HOCl] = constant * FC / [CYA]

where you can clearly see that if the CYA concentration is doubled (for the same FC level) the active chlorine (HOCl) concentration is cut in half. There is no "limit" in terms of some kind of saturation point.