I am not so into synthetic meat ;-) So this is all speculation from my side. You can grow these cells for many cell divisions just not forever. So you keep a master stock of an early generation in the deep freezer (maybe 1000 vials). Then thaw one vial and expand the cells. Maybe you create a secondary derived stock. If they are old, thaw a fresh vial etc. If the/master stock is used up. Then you need to generate a new one.

P.s. regarding the transmissible cancer cells. I don’t think it would happen likely. I think the cells need to loose the MHC gene/protein for this. Just wanted to tell that it exists.

P.p.s I would be much more worried by viruses/ mycoplasma unknowingly infecting the cell culture. On the other hand farm animals are sick all the time too.