Monitoring number of backends

Hi all.

My website is about to get a little more popular.  I'm trying to add in
some measurements to determine an upper limit of how many concurrent
database connections I'm currently using.

I've started running this:

SELECT sum(numbackends) AS count, sum(xact_commit) as ttlcommit FROM
pg_stat_database;

Every 10 seconds or so.  I don't think its what I want though.  It seems
way too small.  I'm guessing that its not a measure of the previous 10
seconds.  Its a count of how many backends are in use at the exact
moment I run the sql.

Is there a cumulative count someplace?

Thanks for your time,

-Andy