On the Boron and Argon there’s currently something like 10K of the 2MB Little FS used by Device OS. I’d avoid filling it completely full in case Device OS needs more of it in the future.
There isn’t a reserved amount for wear leveling, however it does affect the life. The 2 MB is 4096 sectors of 512 bytes. If the flash is nearly empty, you can write nearly 4000 sectors before a sector is reused, so the wear on any single sector is very small.
However, if you only had 10 free sectors, every 10th write would have to erase and rewrite one of the sectors. But if the data rarely changes you can still safely fill most of the flash memory.