99 isn't that much if you think of it in a different aspect that 99 bettas each in individual 3 gallon tanks...
For example, you could easily have a custom built tank that is 50 gallons long divided into 2 gallon sections that is built with a custom stand to hold 2 of these tanks. That is easily 50 bettas right there. People do this all the time. Think of the barracks members make on here.
Or perhaps a 55 gallon (normal tank) sorority with 45 girls in it. Maybe someone really likes females and has 2 of these set up and then WHAM you're at 90 bettas already!
But when you said "The biggest tank is 1 gallon" I'm believing they could possibly be in a bad situation.. That's the largest? I understand if a bunch of them were in 1-1.5s and had some up to 2.5 gallons, but that being the largest is kind of... scary?
There are so many ways to make betta keeping easier... You could have drainage plugs for water changes, no substrate tanks, large barrack systems, etc. so don't judge when people on here have 30+ bettas. I can honestly say all of their "babies" are loved and have very lovely tanks and all are well-cared for. Just because some of us may be overwhelmed with that many, doesn't mean others are!
Kaida- CT Female 2.5 Gal.
Julep- HMPK Male 2.5 Gal.
Fandango-HM Male 10 Gal.