For the best answer, ask the manufacturer of the panel. (Get it straight from the horse I say!)...
I have a 100A Siemens panel with 20 breaker positions. It currently has 2 tendem breakers installed.
I want to add a few extra circuits in my basement. How many additional tandem breakers can be added?
Need the model number of the panel, to answer that.
Example: Siemens model #G3040B1200CP.
Has 30 spaces (#G3040B1200CP)
Can have 40 circuits (#G3040B1200CP)
... so, a max of ten tandem breakers.
Master Plumber Mark:
there is nothing better than the
manly smell of WD 40 in the air
while banging away on brass with a chisel and hammer...
it smells like......victory......
do not hit your thumb...
Just so everyone's clear: I'm the POODLE in the picture ("french", get it?) The hot woman is my wife.