Limitations of the Self-Incompatibility Mechanism in Plants
Herrera, Sara et al. 2018 suggest that there are at least 33 S-alleles in apricot.

I believe some species of Brassica have at least 60 S-alleles identified.

However, there are dominance relationships among these alleles, so I don't think you can directly extrapolate from the number alleles to to the rate at which non-self pollen will be excluded.  The frequencies of the different alleles within a population will clearly also affect this calculation.

In addition, different individuals with the same S-alleles are more likely to be closely related, so I would hesitate to classify this as a limitation of the system.

Ref: Herrera, Sara et al. “Identification of Self-Incompatibility Alleles by Specific PCR Analysis and S-RNase Sequencing in Apricot.” International journal of molecular sciences vol. 19,11 3612. 15 Nov. 2018, doi:10.3390/ijms19113612