The model number you listed gave all the information you need as to the panel and its # of circuits. It is a main lug 12 space 24 circuit single phase 125 amp panel. If every single space has a tandem or twin single pole breaker in it you will have 2 x 12 = 24.... 120 volt circuits. Any space above the bottom two double pole breakers for the oven and cooktop can have tandem breakers installed. So all those 20 amp single poles can be replaced with tandem breakers like the 15's. Only be sure to install 20 amp tandems. Like the others said I'd check to see if that tandem 15 that isn't labeled is being used. My guess is yes since it is in the 'on' position. Don't be to quick to critisize your electrician there are usually a lot of reasons why something may or may not have been done. If that tandem 15 is being used you still have 6 single pole circuits available to you using tandem breakers.
Tandems are designed and listed for use in this panel so they are fine to use and will perform as well as any other breaker.
Murray tandems are common.... you should be able to get them locally just about anywhere.
So what you're saying it that I can replace the fat circuit breaker with 2 slim ones?
I believe I answered this above. The two bottom breakers are double poles, they use two full spaces
. The 20 amp breakers are full size single pole breakers that you see in your panel and use one full space. So one tandem will fit in one full space giving you 2 branch circuits at 120 volts each.
So you need to tell us what voltage or appliance is required for your new circuit then we can determine if you need a tandem or a double pole breaker. You will be able to do either.