I would agree, at 20 mph, about 30-40 ah. My answer based on a crapton of 30-80 mile rides.
20 ah will get you 30 miles at 20mph, To have a huge reserve left at 50 miles, more like 40 ah of 48v.
This type of ride is why one of my custom bikes was designed first and foremost, to carry two 48v 20 ah batteries. To have enough to do 80 miles at 15 mph.
Ride with a watt meter, and know what wh/mile average you need to hit your range. Then adjust your speed as needed out on the road to hit that number, whatever the weather or terrain.
So say you have about 2000 wh, 48v 40 ah. 80% of that would be 1600 wh. You would need to get 32wh /mi average to go 50 miles. 20 mph should be more like 20-25 wh/mi, but depending on the weight of the bike, ( 30 pounds of battery for starters) and the terrain, and the pedaling effort you can do for all 50 miles, 32 wh/mi by the end of the day is very plausible.
This is why I don't give you a promise you can do 50 miles on 30 ah, even if its possible. Unless you ride it slower, which increases the % of the total watts you get from your legs, per mile. Most of us at 20 mph, only produce about 20% of the energy used with our legs. most of us can't put out a 150w effort for 50 miles. Bike racers put out 200- 300w for 150 miles!!!
Anyway, slowing to 18 mph vs 20 mph, will make an astonishing distance possible from 30 ah. When I needed to make it to a town 60 miles away, I'd slow to 15-18 mph. Then maybe, if I did well, I could speed up the last 10 miles.
The tool you need is a cycle analyst, it will read out your average wh/mi. Then if you see 25 wh/mi at mile 40, you can rip it in full speed the last 10 miles.