There was one person assigned to the school from the Sheriff's department: Neil Gardner.
Source: Investor's Business Daily
Gardner was joined by motorcycle patrolman Paul Smoker, who had been writing a traffic ticket nearby.
Source: Wikipedia
By my definition, that is one armed guard at the school. Smoker was not in any way assigned to the school. He was simply the closest regular police officer to respond. You may of course count things differently.
Just to clarify things in context in which this claim is often being made (that armed guards were present, ergo, the idea of having armed guards or teaches is not helpful), the linked IBD article explicitly states:
> One thing is certain — **the armed resistance of Gardner and his backup bought time and saved lives**.