I have been working on a catalogue of Messier Like Objects.
The faintest open cluster in Messier's catalogue is magnitude 8.0. (M26)
The faintest globular cluster in Messier's catalogue is magnitude 9.2. (M72)
The faintest planetary nebula in Messier's catalogue is magnitude 11.0. (M97)
The faintest galaxy in Messier's catalogue is magnitude 10.1. (M91 and M98)
That is according to the SAC database.
http://www.saguaroastro.org/content/downloads.htm
If we use the above magnitude limits and SAC v8 we get 288 OC, 79 GC, 50 PN and 133 Gxy brighter than those magnitudes.
If we use a different database the numbers will be a bit different.
But you need to take other things into account too.
For open clusters the number of stars and richness need to be considered. I don't want 288 OC, it is too many.
Globular clusters are fairly straight forward and 79 is a good number.
For planetary nebulae size is a factor. The smallest is M57 (86" diam), should we go smaller than that?
There are only 7 non-Messier PN brighter than mag 11 and larger than 80", and 24 non-Messier PN brighter than mag 11 and larger than 18".
For galaxies we need to look at surface brightness. (M101 has the lowest SB)
And the hard one is nebulae, how faint should we go?
Which NGC and IC nebula are brighter than M1, M8, M16, M17, M20, M42-43 and M78?
The smallest Messier nebulae are 8' across, should we go smaller? M1 and M78 are the smallest and faintest M nebula.
What do you thinK?