It's not really how many outlets you can put in one run. It's how many things will you have plugged in to that 15 amp run. wattage divided by voltage will give you how many amps you will need for that run. If when you figure all your wattage out and you're close to that 15 amp mark then you'll want to consider putting in a 20 amp breaker. Just remember 15 amp is #14 wire and 20 amp is #12 wire. If you're unsure what you might have in the future just be safe and run 12 wire. If you just want to know cause you want to add an outlet in an existing run then figure the wattage and divide by the voltage(120) and that will give you what amps you are currently using for what you have plugged in. Usually outlet runs are no where near the 15 amp rating for the breaker. But again.. you never know...