The sun flips its magnetic poles every 11 years or so (north becomes south, and south becomes north). During that flip (it takes about a year to complete the flip), the magnetic field throughout the sun is reconfiguring itself, and so sunspots are very rare.
Once the flip completes, then there is a good few (4-5 years, sometimes more, sometime less) of stable global magnetic field and during that time it is much easier for more bigger sunspots to form.
Then a few years later, the poles start to flip again, everything gets reconfigured, and the number of sunspots starts to to dwindle once more.