"dozens if not hundreds of" means definitely dozens, and perhaps even hundreds, although I can't say for sure. The speaker is emphasizing it was a large number, perhaps as many as hundreds, with a reservation about that larger figure. There are other such uses of "if" which might be useful to know. - I have done most, if not all, that you asked me to do. (at least most of it, and perhaps all) - I seldom if ever sleep during the daytime. (I very rarely do it, but I can't say I never do it) - I might possibly get a thousand dollars for my old car, if that. (might get that amount, but even that is doubtful)

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if not 'hundreds' then 'dozens' The person has met dozens of people and is sure about this fact. He also admits the possibility that the number of people that he met could be hundreds.