Well, think about it. There is a maximum of one coin warp per level in the real game. Any more would sort of spoil it, you know? Overusing certain cool things makes them not as cool anymore (common problem among submitted levels). My advice is not to put more than one coin warp in any given level.

The only reason that I can think of that you'd want more than one is in creating a level in which the coin warps are essential to the plot of the level. What plot that would be, I dunno.