How many judges should be involved in the rating process?

This depends on the number of applications you are including in the rating and selection process. Applications are assigned to judging groups, therefore all judges within one group are rating the same applications. You should have a minimum of 3 judges per group so that the final score per application is based on the aggregated scores of at least three judges.

The more applications you have, the more judging groups, ideally. if you have 50 applications in the rating process, great if you have 5 groups of 3 judges each, so each group rates 10 applications. The more applications per group, the more time you are asking each judge to contribute.