Ok everyone please don't list them by bichir species when sizing up the min tank size but bichir sizes instead. As not all bichir grow the same. I keep reading that sene or most lower species such the palmas complex, retro and moke are good for 55g-75g. There are sene that can get up to 13"-16" as well as some palmas complex to 20". Also it not how many bichirs can be keep in a certain gallons of water but how much bio-load can your filter can hold.
1" or smaller 3-5g+.
2"-3"+ 10g-20g+
4-6"+ 25g-38g+
7-9"+ 40g-55g+
10-12"+ 60g-90g+
13"-15" 100g-120g+
16"-18"+ 125g-160g+
20"+ 180g-200g+
(This is just my own. Other may have different views. Always remember the bigger the tank the better.)
You can keep as many as you can but make sure the size are almost the same and not one is being endanger of being eaten. Also the most important is each bichir in the tank can get it share of food. As well the filter must be able to hold the bio-load and never forget the regular water changes.