Hello and welcome @thedan100
Kicad does not have a 10mm mounting hole. You will have to make a hole, modify an existing hole, or import a hole from a third party.
I have no idea how familiar you are with PCB designing or Kicad, so I will start as if you are new to everything.
A 10mm hole will be a snug fit to a 10mm item. Maybe you need some tolerance? A 10.5 mm hole will be an easy fit tolerance.
If you are importing a hole from a third party, you will need a personal library in which to place the hole before it can be used. If you create the hole it is Kicad best practice to save the hole in a personal library.
If you have a personal Footprint library, good. If not, use these links to create a library:
https://forum.kicad.info/t/kicad-7-beginners-guide-to-personal-symbol-and-footprint-libraries/38738
https://docs.kicad.org/7.0/en/getting_started_in_kicad/getting_started_in_kicad.html#tutorial_part_4_custom_symbols_and_footprints
When you have a personal Footprint library, it only takes a few seconds to create your 10mm hole by modifying an existing (8.4mm) hole.
Open your Footprint Editor, scroll through the LH libraries to find a suitable footprint to modify.
I chose “MountingHole_8.4mm_M8”.
Next: Right click mouse, choose “Save as”
In the newly opened box, in the name, change the 8.4 to 10.5 (or what you wish) and the M8 to M10.
Next: scroll through the list of libraries in the same box and highlight your new personal footprint library, then click Save.
Next, scroll through the LH library list 'till you find your personal library, open it and highlight your new 10.5 named hole which is still 8.4mm diameter.
Next: Right click the filled blue circle and select “properties” from the list.
In the properties box, change the TWO diameters that show 8.4mm to 10.5mm and click OK.
If you wish to change the “User Comments” or “F. Courtyard” circles, right click each in turn to show their properties panel, then modify the Radius to suit yourself.
All done! You now have a 10.5mm mechanical hole footprint you can use on any project you wish.
Once you have personal libraries, it is often far quicker to make or modify symbols and footprints than it is to scour the internet looking for something that may, or may not, need altering to use with Kicad.