  We present a detailed method for accumulating Ca$^{+}$ ions controllably in a
linear Paul trap. The ions are generated by pulsed laser ablation and
dynamically loaded into the ion trap by switching the trapping potential on and
off. The loaded ions are precooled by buffer gas and then laser-cooled to form
Coulomb crystals for verifying quantity. The number of ions is controlled by
manipulating the trapping potential of the ion trap, partial pressure of buffer
gas and turn-on time of the entrance end cap voltage. With single-pulse laser
ablation, the number of trapped ions ranges from tens to ten thousand. The
kinetic energy of loaded ions can be selected via the optimal turn-on time of
the entrance end cap. Using multiple-pulse laser ablation, the number is
further increased and reaches about $4 \times 10^{4}$. The dynamic loading
method has wide application for accumulating low-yielding ions via laser
ablation in the ion trap.
