What percent of genetic disease are preventable?
There is no simple answer to either of your questions.

There are many monogenic diseases where mutations in single genes result in disease phenotype.

There are far more diseases that have a genetic component where mutations in one or more gene increase the baseline risk of disease and this risk is then modified by the environment the individual encounters throughout their life-time (see Multiple Genes).

To get an overview of the functions of genes that have been so far identified, mutations within them and the diseases they have been linked to see Online Mendelian Inheritance in Man (OMIM).

If you want to get a feel for just how complex genetics is then a recently published paper on the genetics of hair colour would be worth reading...

Genome-wide study of hair colour in UK Biobank explains most of the SNP heritability