Drone Batteries
Unless you're building a drone, your battery options will be limited by the manufacturer. Most drones can use batteries that offer a variety of total flight time, but your choice will still be limited by your drone's maximum takeoff weight.
Are you into FPV racing? Look for a higher C rating your LiPo will need to offer a higher discharge rate to get that extra speed boost. For longer flight times, large capacity batteries can be useful, but again, longer duration means a heavier battery and swapping can become a wash.
It depends on how often you fly and what kind of mission you are running.
Cells and Voltage
LiPo batteries are assembled using rectangular cells that are connected to produce a specific voltage. One cell represents about 3.6V; 2 cells, 7.2V etc. The number of cells for your ready-to-use UAV will be determined by the size and design of the battery compartment. DIY pilots will of course have more options
Obviously, the more cells, the higher the voltage and the longer the flight time. As an example, a DJI offers a four-cell LiPo battery for the Mavic 2 at 14.4V for a flight time of 31 minutes.
Capacity
The capacity, usually measured in milliampere hours (mAh), determines how long the battery can power the drone's motors. Easy math - the higher the mAh, the longer the flight. But there's a trade-off - higher capacity means a heavier battery. Another benchmark - DJI offers a 15.2V LiPo battery for the Phantom 4 with a capacity of 5870 mAh and weighing about a pound. This will mean approximately 30 minutes of flight time.
Discharge rate
The discharge rate of a LiPo battery determines the energy extraction rate that powers the drone's motors. This may be one of the most important terms to understand - a drone motor that draws in too much water (and damages the battery) is going for a dirt nap.
Discharge rates are measured in degrees C. As with most LiPo terms, a higher number equals a heavier battery. Once you know the C rating, you can calculate estimated flight times by first finding the battery's maximum continuous amp draw (A).
To do the calculation: Multiply the degree C by mAh (converting a four-digit number to two). This will provide the "A" amp draw of the battery.
Example: A 10C rated 5870 mAh would have 58.7 A (amp draw).
Once you have these numbers, you can connect them to an online calculator to estimate the flight time of your LiPo battery.