Have I used "whom" correctly in the following sentence? Rarely a day goes by here that I don’t witness this core value manifesting itself in a myriad of thoughtful, sincere, acts of kindness toward our residents, many of whom have enriched our lives beyond measure.
You use whom as an object and who as a subject.
At first glance, it may appear that in the sentence "many of whom have enriched...", whom is acting as a subject, but it isn't.
In this sentence, whom is acting as the object of the preposition of in the phrase "many of whom", and "many of whom" is the subject.
So, whom is the proper form here.