Grab a pita chip, carrot, or green pepper and test it out. You're looking for a consistency that's easy to dip and won't run. Think: "thin peanut butter" and you'll be good to go.
If you need to thicken your creation, grab some more chickpeas. If you need to thin, use some of the drained liquid you saved from earlier. Try not to use water because it will likely take away from the flavor.
If you think you'll want to back off of the spiciness a bit, use one serrano pepper.
Safety note: During prep time, don't touch your eyes - especially when handling the serranos. Pepper induced tears will ruin any appetizer and these peppers are typically pretty hot.