Consumer SSDs don't have a a lot of overprovisioning. For example, a 1 TB SSD will never have more than 1 TiB of flash. Server SSDs are a different story.
> a 1 TB SSD will never have more than 1 TiB of flash
It's a bit more complicated than that. None of the quantities precisely correspond to the definitions of 1TB = 1000^4 or 1TiB = 1024^4 bytes. A "1TB" drive will have a host-accessible capacity of 1,024,209,543,168 bytes.
The NAND chips on a consumer 1TB drive will collectively have a nominal capacity of 1TiB (1,099,511,627,776), but that's more of a lower bound; the actual capacities those chips add up to will be higher. If we assume defect-free flash and count the bits used for ECC in order to get an idea of how many memory cells are physically present, then we get numbers as high as 1,335,416,061,952 bytes for our 1TB drive. If we don't count the space reserved for ECC, then we're down to about 1,182,592,401,408 bytes on defect-free flash, and 1,172,551,237,632 after initial defects (taken from a random consumer TLC drive in my collection).
So that means the SSD is starting out with about 14.48% more capacity to work with than it provides to the host system—considerably more than the 9.95% discrepancy between the official definitions of 1TB and 1TiB. Of course, that 14.48% will be reduced as the drive wears out, and the low-grade flash used in thumb drives and bargain barrel SSDs from non-reputable brands will tend to have more initial defects.
However, every SSD out there has unallocated space. This is for trim and drive write purposes.
At the mfg level, if you needed to get more space, you can. This will affect the drive rates per day (DWPD) rating of the drive.
In the server world, you can get “high endurance” drives that are 90% just under-provisioned for storage space.