The Odoo installer for Windows comes with its very own python setup. I'm quite sure, python.exe is not the path you need, unless you have set up your PATH environment variable accordingly.

Navigate to the directory in which you've installed Odoo, by default it should be something like C:\Program Files (x86)\Odoo 18.0...\.In there you should find a python directory and ultimately a python.exe and a pip.exe, hence your command would probably need to look something like this:

"C:\Program Files (x86)\Odoo 18.0...\python\pip.exe" install numpy

Note: It's a while since I've been running an Odoo setup on Windows, so the exact path will need to be found by yourself.