Count the number of poles and zeros; if the numbers aren't the same then the larger number is the filter order. Don't forget to count multiple poles or zeros with their multiplicity. Actually, you always get the same number of poles and zeros but on a pole-zero plot you might not see them all because some could be at infinity or at the origin.
E.g., in the following pole-zero plot you see 12 zeros, 6 poles away from the origin, and a pole at the origin with multiplicity 6. So the total filter order is 12. (This is an IIR low-pass filter with approximately linear phase in the passband):
