I have a newer(About 10-15 years old max) 100amp panel that was installed by a licensed electrician but with only 6 or so breakers. The older home has limited lighting and outlets so my plan is adding circuits circuits in particular splitting the existing circuits and adding a few items here and there.

Currently all lighting for the second floor, stairways, basement, and a handful of outlets are on 2 - 15amp breakers distributed through the house through a 12/3 then spliced into knob and tube. Resulting in 2 - 15 amp eventually becoming 2-15amp and 3-20amp.

Its a pain dividing everything up but since I am adding more breakers than were originally there is there a limit as to how many breakers you can have in a 100amp service panel even though each new circuit is nowhere near its 80% advised load of which its capable.