I would suggest 2-4. 4 is a tight squeeze in an 18 gallon, I can't imagine why they make 6-8 netpot dwcs in 18 gallon tubs, those must be for yuppies who are actually growing herbs and spices, etc.
it depends on how you plan to grow them,I had 6 plants in one on my first grow and I can tell you thats way to many,I would go with 1 -2at the most unless your growing sog then you can put in more depending on veg time