Any clan, team or individual may only have 2 challenges out at the same time. Once you have completed both said challenges, you can then send out up to 2 more. The max you can have out at any one time is 2 challenges in any section; Clans, Doubles or Singles.
If you already had out 3 or more challenges before this then don't send out anymore until you finish all of your battles. Once you finish your current battles this rule will take effect for you.