Why do I have the message "You are currently playing the maximum of 6 concurrent games for non-subscribers" when in reality i've only 5 active games?

There is apparently a bug in site programming that causes an occasional error of 1 in game number count. If you "send feedback" (link at bottom of every page) the admin people should fix it for you.

Problem solved
Thanks

Why if I have one active game going does a challenger get the message I have no more openings?

Go to the tab at the top of the page for "My Settings" and then to "My Challenge Settings". You will see a box giving your maximum number of games for challenges. Yours is probably set at 1, which means that no-one can challenge you if you already have that number of games in progress. Set it to a higher number.