  Chiral magnetic textures with non-trivial topology are known as skyrmions,
and due to their unique properties they are promising in novel magnetic storage
applications. While the electric manipulation of either isolated skyrmions or a
whole skyrmion lattice have been intensively reported, the electric effects on
skyrmion clusters remain scarce. In magnetoelectric compound Cu$_2$OSeO$_3$, a
skyrmion cluster can be created near the helical-skyrmion phase boundary. Here,
we report the in situ electric field writing/erasing of skyrmions in such a
skyrmion cluster. Our real space/time image data obtained by Lorentz
transmission electron microscopy and the quantitative analysis evidence the
linear increase of the number of skyrmions in the cluster upon the application
of a creating electric field. The energy needed to create a single skyrmion is
estimated to be $\mathcal{E}=4.7 \times 10^{-24}$ J.
