Re: Clarification: only on action card at a time?
You can play as many action cards as you want in one area but your opponents get a chance to play cards in between if they want. So if I have two +4's and need them both to be winning technically I play one, everyone passes, then I play the second - and people can now respond to the new order of things. That is the way the IOS app works.
However - in practice, around the table, you can usually (always?) play all you want at once and it won't change anything - and will speed the game up.