/* Create sample data */

data old;
  input state $ accttot;
datalines;
ca     7000
ca     6500
ca     5800
nc     4800
nc     3640
sc     3520
va     4490
va     8700
va     2850
va     1111
;

proc sort data=old;
  by state;
run;

/* To get the number of observations in each group of states, start */
/* a counter on the first observation of each BY-Group. The last    */
/* observation in the BY-Group contains the total number of  	    */ 
/* observations and is output to the data set.                      */

data new;
  set old (drop= accttot);
  by state;
  if first.state then count=0;
  count+1;
  if last.state then output;
run;

proc print; 
run;

/* Alternative approach using PROC FREQ to generate the same output */

proc freq;                                                                                                                              
 tables state / out=new(drop=percent);                                                                                                  
run;

http://support.sas.com/kb/24/595.html

Determine how many observations there are in each BY-Group by using BY-Group processing.

Counting the number of observations in a BY-Group

SAS