How many US District Courts are there?

There are a total of 94 federal Districts, but only 90 of them are in the 50 states and District of Columbia. The other four are in Puerto Rico, Guam, the Virgin Islands, and the Northern Mariana Islands.
There are actually 94 US District Courts (90 in the 50 states and District of Columbia, and 4 in US Territories, such as Guam, Puerto Rico, and the Northern Mariana Islands) and 13 US Court of Appeals Circuit Courts inferior to the Supreme Court of the United States.

Congress determines the number of District Courts.
There are 94 US District Courts in the federal judicial system. Although the US Supreme Court is head of the Judicial branch of government, it is not quite accurate to refer to the District Courts as "belonging" to the Supreme Court. The 94 US District Courts are "below" the Supreme Court, but they are also immediately below twelve of the thirteen US Courts of Appeals Circuit Courts.

The thirteenth Circuit Court, the US Court of Appeals for the Federal Circuit, doesn't have territorial jurisdiction over the District Courts, but has nationwide, special jurisdiction over certain types of cases, such as monetary claims against the United States.