I would guess the more the better, but which tests would you consider an ABSOLUTE necessity, and which could a new puppy purchaser let slide - any? I'm picky, and since researching I would want Fanconi, IPSID, Progressive Retinal Atrophy, Hemolytic Anemia, Hip Dysplasia, Thyroid and PPM. I'm getting all of these questions from reading and from the links on this forum - thanks to those who have provided all of this great information.
Hemolytic Anemia is pretty much gone from the breed, so in looking at the parents, as long as either they or their sires/dams have been DNA Clear, you would not have to worry about HA, as it is a recessive gene and since this is a direct test, offspring are clear by parentage. PPM would be determined with the same eye exam as PRA.
There is no test for IPSID, that is where the knowledge of the dogs by the breeder is so important along with Progressive Retinal Atrophy, again there is no test and it is late onset, sometimes as late as over 10yrs or as early as 3yrs. Again important for the breeder to know the background.
Hip Dysplasia, you again need to look at the sire/dam, offspring/siblings and their test results.
So, I would agree, at minimum, DNA for Fanconi, Current CERF Exam, OFA for hips and the more in the pedigree that has been OFA'ed for hips the better.