Do you want them to be able to bring them back to him one at a time? Or to have him send you away until you've got all 5?
If the first, when he asks you for slimes, set a variable to 0. Each time you talk to him, see if you have a slime in your inventory. If you do, use Control Variables to determine how many slimes you have. Then add that to the variable you're using to count, and remove them from inventory. Then do a conditional branch to see if the variable is 5 or more.
If the second, when you talk to him, use Control Variables to determine how many slimes you have in your inventory. Then do a conditional branch to see if that variable is 5 or more. If it is, remove 5 slimes from inventory and turn on a self switch (if it's ONLY the NPC event that will be changed) or a regular switch (if you want to change the NPC event along with others when you have enough slimes).