why do they only send one ambulance ??

It would probably cost too much to show multiple ambos, crew, etc. I always imagine them instead. Most of these shows demand a suspension of disbelief.

Put puppy mills out of business: never buy dogs from pet shops! 

usually each firehouse only has 1 ambulance...if they need more medical manpower than they would have to dispatch ambulances from other firehouses which would take longer to arrive to the scene. Since this show is focused mainly on firehouse 51 specifically, usually we only see 1 ambulance crew, which is Ambo 61 that's assigned to firehouse 51.

When the call came in to dispatch, their may have been a report of one injury. Units have the capability of caring for 1-3 patients, depending on severity.

Speaking as a Paramedic whom has worked out of Chicago, each house usually only has one ambulance per shift, maybe two depending on the district. "Bad" areas have two, maybe three, "good" only have one.

Usually only the closest fire stations are called and most only have one ambo. If they need more than one, they'll call for one. It's all about saving money which is ridiculous because if you have to prioritize between two critical patients with only one ambo, it's almost guaranteed the one whom is deemed better/safer will die because another ambo needed to be called