minimum of four tools for tool changer just seems to be an evolutionary inevitability to me. For two tools, IDEX is much easier to build and use (and you can make tool changes in less than two seconds with careful tuning). For three toolheads, its just not worth the added complexity and cost of machine space so four is the viable minimum and six/seven the viable maximum without enlarging the machine just to get more docking area ...
(yes, you can get a PrusaXL with one or two tools, but thats a business consideration).