What is the limitation of number of rungs can be used in Modicon M221 PLC.
There is no such limitation of number of rung or number of POU in application. As long as you are having the application size limitation 256kb, you can create as many rung or POU as you want.
The only limitation is the Task type. In SoMachine Basic there is only one Mast task available, One Periodic Taks and Four Event task based on Physical Input and Four Event task based on HSC function block.