I have a 21' EGO mower, the one with the self-propelled feature. Had it bought for me as a housewarming gift from parents approximately 2 years back. I believe the battery is 7.5 amp/h, but I'm not 100% sure. I just have the one battery, and I find that it is almost always able to do my entire quarter acre yard. I mow once a week typically, though there was a period last month where it rained for almost 2 weeks straight, so by the time I was able to mow my lawn had gotten super high. Ended up having to raise the mower deck up as high as it would go, then go over it again the following day on the 3" setting. I typically only use the self-propelled feature when going uphill. If/when my battery ages to the point that it can no longer get the job done in a single charge, I'll probably go out and grab an EGO brand string trimmer to replace my craftsman one, and just use the battery from that to make up the difference.
A couple things that have given me battery life issues, and how I resolved them:
1. I was having a lot of trouble with the side discharge chute getting jammed, resulting in grass piling up around the blades and causing the mower to get bogged down a lot, draining the battery significantly if I didn't keep stopping to clear the clumped up grass out. I resolved this by getting rid of the discharge chute entirely, drilling a small hole in the bottom of the mower's back flap, and using a length of paracord to hold the flap about halfway open. This is technically not 100% safe, but I have been doing it that way for over a year now and have yet to get smacked in the shins with anything. The motion of the blades means that the grass is still getting chucked off to the left side anyways, so it's not like it's getting thrown at your feet. Using this "hack" I'm able to run the mower deck a lot lower without the mower getting bogged down. I can also get away with mowing in the morning when the grass is still damp and it's generally not a problem. Overall this helped my battery consumption a lot
2. At one point I accidentally dropped the battery on the concrete floor of my garage, and after that the battery wouldn't hold a full charge. It would drop to 60% almost as soon as I started the mower up. Over the winter I took apart the battery casing to see if some of the cells had gotten knocked loose, but everything looked fine. Put the battery back together and now it's suddenly back to holding a full charge. No idea what changed, but I guess sometimes taking something apart and putting it back together again can fix the problem!
3. I checked the blade the first time I mowed this season and noticed that it was pretty dinged up. I don't always do the best job of clearing sticks out the yard before I mow (I also have a walnut tree and sometimes I hit fallen walnuts), which I assume is the only reason it was in such rough shape. I sharpened the blade up and it's cutting like new now. Not sure how much of an impact that has had on my battery life, but it certainly didn't hurt.
Hopefully you find some of that helpful. Maybe in the morning I can take a picture of my makeshift discharge chute hack and post it in case anybody wants to copy it.