Zero. A file type doesn't need to be “acknowledged”.
Imagine a photo of a tree. How much surface on the photo is needed to acknowledge where the photo was taken? Zero. The photo doesn't need to include an indication of where it was taken. It may do so, of course (for example, someone could have held a GPS's display in front of the tree), but equally it may not. There may be a legend below the photo, or here may not. And even if there's an indication, it could be false.
This metaphor works for a file's format.
Data in memory is just a bunch of bits. The semantics of this bunch of bits is defined by the code that manipulates it, not by the values of the bits alone. See How does a computer determine the data type of a byte?
If a location in memory may contains $n$ different file types, and the program needs to store an indication of the file type, then storing the type requires at least $\lceil \log_2(n)\rceil$ bits. This is because $k$ bits of memory can store $2^k$ different values; $\lceil \log_2(n)\rceil$ is the minimum value of $k$ such that $2^k \ge n$. In practice, it is common to use more bits (e.g. a whole word), because manipulating arbitrary groups of bits is not easy in a typical programming environment.
This is assuming that the type indication uses a constant amount of memory. If the type indication uses a variable amount of memory (e.g. a string), then the minimum is again 0 (but you then have the problem of how to store the length of the string). When data is stored in files, it's common to indicate the file format via the filename extension. But this is purely a convention; there is no mathematical relationship between a file's name and a file's content.
The JPEG format, like many other file formats, starts with a magic number. It's a 2-byte magic number. If a file begins with the two bytes (0xff,0xd8), then there's a good chance that it's a JPEG file. But this too is purely an engineering consideration — a convention that most programmers follow. It's a social observation, not a scientific observation.