The breakers and the rating of these have limited value in determining the loads present on the panel or whether you can add more loads. Things like motor loads could have a breaker much larger than the actual loads as an allowance is made for surge or startup current that is very temporary short duration and would have less impact on the panel as a whole. Another thing that can happen is lighting loads that had new LED installed and actual loads significantly reduced but breakers is still the smallest of 15A, but load might actually be only 2 to 3 amp total. (Had one we replaced old troughers with new LED ones, went from needing 10 circuits to handle the lighting load to using 3 and probably could have been 2 but customer needs control for seperate spaces where such that 3 was used.)
I did a new 5,000 square feet, 2 story and a half house 5 or 6 years ago that had over 65 recessed lights plus all the other lights throughout the house and garage. We'll over 100 lights total and everything LED. Had a 400 amp service and I checked all 4 legs with every light on the property on and no leg has more than 2 amps. If every leg had 2 amps, the whole house was just using 8 amps and could be on one 15 amp breaker. I didn't do that of course but was surprised of how little current was being used for all the lights on at the same time.