What version of Windows can I install on a 2009 MacBook Pro? I would like to convert my old 2009 MacBook Pro into a Windows machine to use some apps that don’t run or do so extremely poorly on macOS.

This MBP supports El Capitan (10.11) as its last version.
What version of Windows can I install on it using Bootcamp?

Thank you very much
Unfortunately it looks like that Mac will only support Windows 7

https://support.apple.com/en-us/HT205016

Even your model does not look like it supports Windows 8

https://support.apple.com/en-us/HT201457

And Windows 7 is End of Life in mid January. It will still work but no more security (or other) updates after that. So it's do-able but not recommended.