For a given plant species, how many types of S-loci exists to prevent self-pollination? I would like to know how often a plant stops fertilization process by other plants, thinking the pollens belong to itself due to limitations of SSI/GSI.
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