The SET statement reads the descriptor portion of CUSTOMER_RESPONSE at compile time and stores the number of observations (the number of respondents) in COUNT. The SYMPUT routine stores the value of COUNT in the macro variable NUM. This variable is available to the remainder of the SAS session. The IF 0 condition, which is always false, ensures that the SET statement, which reads the observations, never executes. (Reading observations is unnecessary.) The STOP statement ensures that the DATA step executes only once.
data _null_;
   if 0 then set customer_response nobs=count;
   call symput('num',left(put(count,4.)));