If you use homemade ones I would recommend at the very least 3, but it's entirely up to you. If you have some extra money lying around you could buy some laylax barrel spacers and that pack of two should work well enough. I have a 650mm 6.03 barrel and I am using 5 homemade spacers inside.
As many as you can put in there pretty much. You want to get as much surface area of your barrel possible to be "spaced." I suggest the one piece wax spacer. Check out the guide for the LRB wax spacer and follow the instructions minus the wedge to bend the barrel. i also suggest when doing this that instead of using hot glue to plug the barrel, you instead use something like a cleaning rod wrapped in electric, or better yet teflon tape to act as a barrel plug/wedge. Saves you the trouble of getting the hot glue out.