Is swearing to abandon all sins subject to a single expiation, or an expiation for each sin committed after the oath?

It is not permissible to fulfill an oath by committing a sin. One must strive against the self to avoid transgressions, for their consequences are grave. If a sin is committed, an expiation for a broken oath (kaffarat yamin) becomes due, and the oath is dissolved. No further expiation is required unless you intended to repeat the expiation every time a sin was committed, or if the wording of the oath implied repetition, such as "whenever" or "if ever" I commit a sin.