In AC19, I believe we have lost the ability to control how many window units exist in a multiunit MU window. Both the fixed window and the awning MU window used to have this option located in the "All Parameters" setting. Now that this window has been removed, there is no way to change this unit option.
AC21 64-bit
Mac OSX 10.12, 4.2 GHz Intel Core i7, 64 GB mem
Mac OSX 10.11, 3.4 GHz Intel Core i7, 32 GB mem