There are very few American Bochurim at Mercaz.
There are, however, Israeli born bochurim who grew up in English speaking homes.
Many of the Bochurim come from a few specific yeshivot ketanot.
It’s a large Yeshiva and sometimes may be not so easy to fit in (even for Israeli’s).
As for the learning, it’s considered one of the top Yeshivot in the Dati Leumi community (along with Yeshivat Har Hamor and Yeshivat Beit El.
Har Hamor is newer, very popular and has set up communities and numerous Yeshivot Ketanot and Talmudei Torah as well as girls schools. (I’m not so familiar with it)
Yeshivat Beit El is a bit smaller, but perhaps has a more cohesive and warm atmosphere. There too there are very few Americans (but again, there are Israel born Americans- English speakers). As it’s in a yishuv, it has more of a community feel and a clear strong hashkafa.
Any of the Yeshivot would be a great learning and ruchani experience as אווירא דארץ ישראל מחכים.
Of course, there are numerous Hesder Yeshivot- KBY, Shalavim, Hakotel and the Gush that have large American programs.
Much Hatzlacha!!!