Calculating how much LOX is lost is going to be an interesting calculation, but I'm not sure we have the data for that. Instead, I'll be focussing on the other part of your question:
Is the loss replaced by tanker trucks or by a liquid oxygen plant near the launch site?
The ball-shaped structures near pad 39A are storage tanks for liquid propellants. It's pumped from there to the pad by pumps capable of pumping 1300 gallons per minute. I imagine the loss per minute would be substantially less than that.
Overview: Liquid oxygen used as an oxidizer by the orbiter main engines is stored in a 900,000-gallon tank on the pad's northwest corner, while the liquid hydrogen used as a fuel is kept in an 850,000-gallon tank on the northeast corner. The propellants are transferred from the storage tanks in vacuum-jacketed lines that feed into the orbiter and external tank via the tail service masts on the mobile launcher platform.
LOX and LH2 storage (NASA)
No tanker trucks are required to refill the loss by evaporation, directly. Naturally, something has to refill the storage spheres eventually and this is done by truck (thanks Organic Marble).
For more on LOX consumption, see this answer. 1kg of LOX is roughly 0,23 Gallon.
A picture of KSC LC-39A with Crew Dragon Demo-2 and highlighted ball-shaped storage tank (one of the cryogenic liquid propellants tanks):