I get this message regularly when I only have one or two cars open. Especially when the card I am trying to open is an additional browser card. Today I tried to go to the link in the sory about Radiotime. I only have 3 cards open, and when I hit the link in the article I get a "too many cards" message.
At least once every couple weeks I have to restart my phone because I get the message even if there are no active cards.
If you are going to use the ability to run multiple cards as a selling point, the device should be able to support more than 2 or 3.