  The process of topological phase transition in a chiral magnetic material
from skyrmion lattice phase to helical phase has been investigated numerically
and experimentally, yet the analytical expression of the evolution of skyrmion
number and emergent magnetic monopole charge during the process is still
waiting to be explored. In this letter, by utilizing the topological current
theory, we show that the change of skyrmion number of a layer in the system is
equal to the net monopole charges that flows through the layer. Based on this
relation, with the help of statistical argument, we derive the analytical
expressions of the skyrmion number and the monopole charge as functions of the
external magnetic field. We find that the evolution of the skyrmion number is
exactly the logistic growth function, and the evolution of the monopole charge
is proportional to the derivative of the skyrmion number of the system. our
analytical results are in good agreement with the numerical simulations in
\cite{2013SCIENCE}. The goodness of data fitting goes up to 99.94\%.
