Keep in mind, a lot of Melkites worship at Latin churches. Heck, I'd say the majority of the local Melkite parish near where I live, aren't even Melkite (at least one family is Greek Orthodox). Many of them are other Byzantine Catholics, and more than a few are Latins. The late Fr. Michael Hull lamented that the Arabs didn't seem to want to be Melkite (he was biritual, and he very much supported both traditions). Abouna was even learning Arabic at time he died. (He already knew some).
Some of this is for historical reason, but, just like I shouldn't be shamed for being Latin, Eastern Catholics shouldn't be shamed for being Eastern. This, has sadly, not always been the case, and a lot of older Melkites may still have the attitude that "Latin is better".
It's not that they're not there, it's just that they're attending church elsewhere. As to the overall number, no idea, and historical migration patterns aren't going to help give you an estimate.