Reading the manual
Number of rows updated: When an UPDATE statement completes execution, the
number of rows updated is returned in the ROW_COUNT statement information
item in the SQL Diagnostics Area (and SQLERRD(3) in the SQLCA). For a
description of the SQLCA, see SQLCA (SQL communication area).
For a description of ROW_COUNT, see GET DIAGNOSTICS. For a description of
the SQLCA, see SQLCA (SQL communication area).
I don't know if any of those techniques are available in RUNSQLSTM. There
are restrictions on where GET DIAGNOSTICS can be ran (RTM).