Ok I know this question has been done to death but I'm still confused...

I'm not sure how many gallons our tank is so I will attach a picture.

We currently have:
2 Guppies
1 Sucking Catfish
1 Baby Molly

I wanted to get some more guppies but I don't know how many I can put in the tank?
You can see the 2 Guppies to the very right of the tank for a size reference.

General rule is 1 inch of fish per gallon of water. So depending on how big you want your fish it's a rule to follow. You can get a general idea of how big the tank is by measuring it and reducing the number to cubic feet.

How many gallons of water in a cubic foot


Also your guppies like places to hide. They can be robust breeders so keep an eye on them.

Oh don't worry they are both males! No female guppies in that tank, been there done that a few years ago!

Ok so I have figured it out (it hurt, but I did it) and it is 7.5 gallons.

Ok so Guppies are (let's be generous) 1 inch each
Catfish is 2 inches
Molly 1 inch

So I can get 2 or 3 more fish? Is that right?

Yes that should be OK. Just monitor the tank and maybe get an airstone in there to supplement the oxygen in the water and to give them something to play in. Anytime you start adding fish just be careful and monitor the amonia levels. Make sure to keep filters clean etc. You should have a nice active tank ;)

The filter blows out bubbles as well as water but I don't have it on all the time, it's too powerful for the tank and it's built in so I can't change it grr.
I will look into getting an air pump for it... we did have an old one here but it has somehow disappeared in the mess of boxes we call a spare bedroom.

I know how that can happen. To control pressure all you need is a valve so you can turn it down and let it run all the time. They make them with 1 or several valves you can use from a single air supply each with its own pressure.

Use the following web site caculator to get your total gals. Just plug in your measurements.
http://www.pondcare.com/reference-center/pond-calculator/index.aspx.