  The response of superfluids to the external rotation, evidenced by emergence
of quantised vortices, distinguishes them from conventional fluids. In this
work, we demonstrate that the number of vortices in a stirred polariton
condensate depends on the characteristic size of the employed rotating
potential induced by the nonresonant laser excitation. For smaller sizes, a
single vortex with a topological charge of +-1 corresponding to the stirring
direction is formed. However, for larger optical traps, clusters of two or
three co-rotating vortices appear in the narrow range of GHz stirring speed.
