Likelihood of getting campsite spots day of?

Hi all,

I’m planning to hike the WHW solo at the end of May. I’m intending to wild camp most of the route (will book in the Loch Lomond permit zone), but I don’t have much solo wild camping experience.

How realistic is it to rely on campsites as a backup and get a spot on the day if needed? Do they tend to fill up in advance that time of year, or is there usually some availability for walk-ins?