As jamesw said, there are a number of possibilities, but we can narrow it down with a little inspection.
Here's a copy of a 8o slate & carmine....
This is a 1875-79 Scott 28 8o slate & carmine with normal frame.
This is a perf 14 X 13 1/2. You will note it has 13 perforation holes horizontally, as does your copy.
be your stamp- Catalogue Value 50 cents.
But there's more.
Your stamp should have this watermark....
Wmk 112-Crown (Step away from the computer monitor a bit to see it well)
There are several other watermarks (wmk 111, wmk 113) that Denmark used during the 19th century, and they all have a slightly different shaped crown. Anyway, your watermark crown should resemble the image above.
But there is more.
The stamps can be found with either normal frames and inverted frames.
Here is a 8o slate & carmine with inverted frame....
1875-79 Scott 28c 8o slate & carmine with inverted frame
Catalogue Value 50 cents
Notice the curly-cue arabesque in the upper left portion of the stamp that directs itself toward the "D" of "Danmark"?
Think of it has an upsidedown tree or plant with various branches: pay attention to wear the branches come off the downward vertical main stem.
Let's compare to the normal frame stamp.
Normal frame versus inverted frame
Look at the upper left side of each stamp...
In the normal frame, a right branch comes off before a left branch comes off. In the inverted frame, the branches come off the downward vertical main stem at the same place on either side of the stem.
Do you see the difference?
Now look at your stamp...which frame do you have?
You should now have a good idea what to look for with these Denmark Numeral stamps.
Finally,just for fun, here is another pic of a 8o slate & carmine...
1895-01 Scott 44a 8o slate & carmine
Perforation 13, watermark 112- Crown, Inverted Frame
In summary, there are three variables to consider with these stamps: Perforation, Watermark, and Normal/Inverted frame.
You can tell, I really like these issues.