How to set automatic values in *.tcl files (automatic maxChannel in my case)

Simpler solution: when you create a custom component using the Qsys Component Editor, on the Parameters tab, set the parameter to Editable. You can also set the default value for the parameter or just use the default value from the HDL code. This way you don't have to edit the _hw.tcl file (unless you need more advanced parameter value checking).