I have 4 and one more would let my fly and get one recharged for the 6th flight without a break (assuming lower than 25% for each flight). 5 is the right number. And we often go thru all 4 taking a break and still barely getting a recharge. Might add a charger, or use the 12 volt charger at the same time. Still, 5 would be better. (and we have a fishing gear bag for our carry, the sling is comfortable, and no one wants to steal a bag of someone's fishing gear.