I made a quick count from what appears to be reliable information, and there appear to be about 507 churches (parishes and missions) which have the EF. Some of those may be once a month; many are not celebrated on a Sunday or a Saturday evening, but rather some time during the week, and not all of those are weekly. For example, there may be an EF on a Saturday morning or early afternoon.
As there are about 17,337 parishes, that puts it at a little over 2.9% of parishes nationwide which have an EF celebrated at least once a month - with many weekly.
Some dioceses and archdioceses may have a greater percentage than that, which of course means that others have less, or none.