To be quite honest, I don’t know. I’ve not tried to find out. It would not surprise me to find a few in the 19-20 range but fainter than 20 are likely to be more common.
That said, I can take measurable images at magnitude 20.5 unfiltered in 51.5 minutes (see https://britastro.org/observations/observation.php?id=20181108_230500_5f60877f698839f2 for an example) and 22.2 in 178 minutes (see https://britastro.org/observations/observation.php?id=20200113_230000_a5cc9ad9ed617011) with a 0.4m on a decent night.
Owners of smaller scopes need to multiply by the square of the aperture ratio. A 30cm, say, takes (0.4/0.3)^2 as long — 92 minutes and 316 minutes (5h16m) respectively. The figures for a C14 (0.356m) are 65 minutes and 225 minutes, or 3h45m.
I accept that over five hours is almost certainly too long for most people. Nonetheless observers equipped with telescopes in the 30-40cm class should be able to do useful research by measuring objects which are significantly below mag 21.