I'm not sure if you can combine input numbers with the formula from the GUI modeler. I tried testing it and didn't seem to like it.
An alternative could be to create a custom script which, when executed, can look like the exact same interface as when you run a model. The difference here is that you have to write out the code (you can find more information here: Writing new Processing algorithms as python scripts).
To create such a script, go to:
Processing Toolbox > Scripts > Tools > Create new script
Then copy/paste the following:
processing.runandload("saga:rastercalculator", Input_Raster, '', 'a / ' + str(Number), False, 7, None)
Save the script and run it from the Processing Toolbox:
Note: I receive errors when trying to save it directly to a directory. To avoid this, the code saves and loads the output as a temporary layer. You can then save this layer manually.
Essentially, you define various parameters at the beginning of the script (i.e. name of script, input layer, input number). The last line calls the Raster Calculator function from the Processing plugin where you can define the required parameters. You can find what parameters are required from the Python Console (from the menubar
Plugins > Python Console) and type:
You should receive:
Use iface to access QGIS API interface or Type help(iface) for more info
>>> import processing
ALGORITHM: Raster calculator
TYPE(Output Data Type)
0 -  bit
1 -  unsigned 1 byte integer
2 -  signed 1 byte integer
3 -  unsigned 2 byte integer
4 -  signed 2 byte integer
5 -  unsigned 4 byte integer
6 -  signed 4 byte integer
7 -  4 byte floating point number
8 -  8 byte floating point number