Usually, 15 reps are ok to begin, and then you can add 5, and 5 until 25 it’s good exercise for your body. But it’s always better to smart small and higher your goals to not discourage yourself ! Cheers ☺️
Don't focus on how many you can do, start slow. Make sure your using proper form, never push yourself to get as many reps as you can in 30 sexonds. As you improve you'll be able to fit more and more reps into that 30 seconds. Instead see the number of successful, well form reps you can do as a marker of your progress.