It's a kind of joke. The stock phrase would be "a plan or two." The speaker means the person is sure to have plans. "A plan or two" would be the normal thing to say. "A plan or five" is a joking way to emphasize it.

typically we might say "he has a plan two".... to indicate more than one plan. Using a number more than 2, eg 5, just emphasises the fact he has plenty of plans

Hi there. Hope you are well. In the text, it just means that he probably has more than 1 plan. The one person is concerned that the other person is aware of what's going on and has a number of different plans. Hope this helps.

Having five plans could also suggest that they aren’t good plans. Having so many could indicate that he didn’t spend time on coming up with a single good plan. But you would need more context to say for sure.