Back when I had saltwater aquariums in addition to freshwater, I had quite a few. I am not doing the counting as you might not count the same way I do.
My largest saltwater system was a 125 gallon tank plus two
sumps.
I had a combination of a 55 gallon tank, a 29 gallon tank, a
refugium and a sump.
I had another system that had a 40 gallon breeder plus a 20 gallon tank and a sump.
For freshwater, I had a 90 gallon community tank and a nano tank for a betta.
So I had 5 "systems" with multiple tanks.
The
saltwater tanks were wonderful, but they were time consuming and expensive. As the fish and invertebrates died, I did not replace them. The marine species are gone now.
After the saltwater phase, some of the tanks were repurposed. I once had multiple freshwater tanks: a 55 gallon, a 40 gallon breeder, a 29 gallon, a 20 gallon high, a 20 gallon long and a 10 gallon. However, as I got older, the maintenance of so many tanks became a burden. I now have only two tanks in operation, the 55 gallon and the 40 gallon breeder. As those fish die, I am unlikely to replace them. However, the population of
platys in the 55 gallon tank is fairly stable. As fish die, new fish are born.
I have kept fish for over 6 decades. It has been the longest lasting hobby in my life. Many other interests have come and gone over the years. Carrying jugs of water for
water changes is becoming more difficult, so I do not know how much longer I will be able to care for my fishy friends.