Yes, whom is correct. For them to be correct, it would have to begin a new sentence. Yes, client is a mistake; it should be plural.
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Son James"We had a series of meetings with important client yesterday,most of ___ are very satisfied with our productA series of meeting - isn't wrong. But "several meetings" may be more common in casual conversation.
Why didn't the questioner write as clients? Isn't it his mistake?