well, they have one to use half a decade from now (2020), two more in reserve, and per your statement, they can make one per decade. Seems they are doing fine!
Could we not reprocess uranium into plutonium from commercial reactors to recycle the waste and use that for space probes?
238Pu is only efficiently manufactured by activating 237Np.
If any nuclear material could be easily turned into any other nuclear material, life would be so much easier, and nuclear waste would be much less of a problem. ;)