What is the purpose of the Carothers equation?
It is an equation useful for describing the behaviour of a large system, not predicting the fate of a few monomers.
Polymerization is a stochastic process—if we only have 80 monomers, the resulting polymer molecules will vary wildly in length if we perform the experiment several times (and logically, could not ever have an average degree of polymerization of 100), and the equation could not provide much insight into the results.
However, 80 monomers is a very tiny quantity.
If we take one of the monomers used to make PET, terephthalic acid, at 166.13 g/mol works out to around $2\times 10^{-23}\ \textrm{kg}$ for 80 molecules, a quantity that is almost impossible to actually measure out so it doesn't really matter that the equation doesn't work.
At a more practical scale, with even 1 g of the monomer, we have ~$10^{21}$ monomers and we will see the average behaviour of the system that the equation describes.