From the MA User Guide:
"The default number of output cells that is generated is set by the NumberofCells keyword in the stored process. The MinCells and MaxCells keywords specify the minimum and maximum number of cells. The NumberofCells keyword does not have to be specified. If the MaxCells value is specified, and MinCells and NumberofCells values are not specified, the minimum number of rows is 1. If only the MinCells value is specified, the maximum number of rows is unlimited. "
several macro variables provided to your Stored Process are useful for understanding the input and output MATables:
OUTTABLE0 and INTABLE0 have the number of OutTable and Intable MATablenames available via OUTTABLEN (where n=1-&OUTTABLE0)