ABO blood group typing via SNPs

Question

[theoharis]

What is the minimum number of SNPs one could use for ABO blood group typing?

I noticed this implementation using 3 SNPs:

https://github.com/rivas-lab/covid19/tree/master/ABO

ABO allele  rs8176746   rs687289    rs507666
A1             G           A           A
A2             G           A           G
B              T           A           G
O              G           G           G