What version of Windows can I install on a 2009 MacBook Pro?
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.