first sync up 4 wii remotes to each system and test to see if they get interference on the 1 wii. then repeat with the others.
after that try 2 wiis at once with all 4 wii remotes active. if there's no interference then try it with 3 and then 4 and 5 respectively. if you get any interference then you know you've hit the max.
as far as i know though the wii syncs up to a specific signal from the wii and the wii generates one of like 6 or 7 signals on its own so there shouldn't be a problem but you should test before hand to make sure cause i'm not 100%