What is the magic number 19, that rebel TMC MPs need and Mamata wants to avoid?

About 20 rebel MPs of Mamata Banerjee-led TMC have decided to support BJP-led NDA. As per rules, if 2/3rd majority of a party decides to switch to another party, they can do so without being disqualified. With TMC holding 28 LS seats, rebel MPs want 19 MPs on their side, while Banerjee wants to cut their number below 19.