Saying a couple of days literally means two days but often people use it less formally and a couple can mean more than 2 but generally less than a week. Again, that would only be very casual, often times when people say a couple of days i clarify with them they actually mean 2 days. Plenty means more than enough, that you have extra, or that it is sufficient. Again plenty is something that could be used more casually to mean just enough. For example if your friends mother invites you to sit down for dinner stating "Sit down and eat, there is plenty of food" this could mean that there would have been extra had you not sat down, but once they add you into the eating group there will be just enough for every person. I hope this helps out!