The concern with antibody testing, in addition to Merry's comment in not knowing what is a "safe" number in terms of antibodies, is that the type of antibodies produced, and the type of test are different between the infection and the vaccine.

Nobody has all the answers yet, and research is ongoing. For people who are immunocompromised, such as transplant recipients, a few medical facilities are able to offer a special test which determines whether antibodies were produced by the vaccine, and at what level. Right now, it is expected that a 3 shot (booster) will soon be recommended for very specific populations.

Here is the most recent statement about antibody tests: https://www.fda.gov/medical-devices/safety-communications/antibody-testing-not-currently-recommended-assess-immunity-after-covid-19-vaccination-fda-safety

As with all things Covid, stay tuned for further developments.
Sue