As the documentation says, UN is used for unknown encoding - all DICOM information is encoded in bytes, but then converted by pydicom to strings, floats, etc. according to the type. In this case, the type is unknown, so pydicom leaves it as bytes.
You will have to figure out what the correct encoding is with some known information. In this case, since it is a count of slices, it is most likely an integer. For example, assuming you have little-endian order and a long integer:
bytes_string = b'h\0x01\x00\x00'
gives 360, which seems a reasonable value as number of slices.
There are ways you could automate this somewhat. For example, if you had multiple UN to deal with, you could look into the documentation for `data_element_callback` to write some code which could check and deal with various UN elements.
Hope that helps