A macro variable created with PROC SQL from a numeric variable or function contains leading blanks:

proc sql; 
  select count(*) into :howmany from SASHELP.CLASS; 
quit; 
%put |&howmany|;

|      19|

An easy way to get rid of the leading blanks is as follows:

proc sql;
  select count(*) into :howmany separated by ' ' from SASHELP.CLASS; 
quit; 
%put |&howmany|;

|19|

Posted with thanks to Robert Allison: Since the original post, the TRIMMED option has been added to the INTO clause in PROC SQL to strip blanks:

proc sql;
  select count(*) into :howmany trimmed from SASHELP.CLASS;
quit;
%put |&howmany|;

|19|

 See the documentation for the INTO clause here.

Thanks to Mike Zdeb for sharing this tip on sasCommunity.org.