They add reserved signs based on demand.
If 10 new drivers pay for reserved spots, the GO staff actually visits the stations and installs additional "Reserved" signage. That's what happens. There is no limit or cap at the moment -- some GO stations (e.g. Long Branch) have become almost entirely Reserved through this process.
If there are no reserved parking spots, you can pay to reserve anyway. They'll come and install the first "Reserved" sign because of your call.
That has happened before... Once a parking lot becomes full for the first-ever times, the first calls to reserve come in, and -- bingo -- first Reserved signs arrive.
Better promoting of this feature could help prevent building new garages -- such as "Tap Your Presto to sign up for a Reseserved Parking Spot This Month!" or other features that make it super easy for drivers to spontaneously obtain reserved parking spots and convert parking lots into defacto paid parking lots faster -- and cut GO fares quicker (e.g. $3 GO fares for those who do not bring a car).
And better promoting of transit interchange opportunities ("Don't want to pay for parking? This GO station is also served by [Gotham Transit #404 #777] [Kiss-N-Ride] [Uber] [Carpool Recruitment Network] [Cycle Network]")
Even cheap simple steps can be accomplished initially. 2 meter tall by 1 meter wide, full color signs mounted in standard placements. Simply to advertise things like "Want a parking spot reserved just for your car? Book one today! gotransit.ca/reserve" + alternate connections (transit routes, carpool signup services, ridehail, GO buses, etc) .... The extra revenues from paid reservations will more than pay for signage.
Or these signs mounted on top of movable blocks at the back of spare unused reserved spots -- and allow drivers to book those parking spots the same-day on the spot (drive in, use smartphone to pay for spot for the whole month, done. Now a GO employee will put a "RESERVED" on that specific spot during the day before you return home from your commute). Make it as easy as possible to reserve parking spots. Reduce the cost for people who do not bring cars.
This may be a useful transitionary technique before the era of fully-paid-GO-parking.