I'm wondering what number of crabs I could comfortably fit in my tank. I want to adopt some more!

My crabs are Aussies, so that's why I'm thinking that I may need more space. I have 3 crabs right now, all around 1.5 inches. I don't believe that they grow as big as most species- is that right? I've seen the chart, but I thought it may be different for Aussies.
I have a 65 gallon Exo terra tank. It has around 6-7 inches of substrate- play sand and eco earth. The humidity is usually 80-85%, and the temp around 25 Celcius. There is loads of climbing and hiding stuff like reptile hammocks, coco huts, sticks, Mopani wood, etc. plenty of shells, too. It is pretty much a second level.
Thanks
