Sadly, there is no standard. Middle C is always note number 60 but it may be described by a different octave designation – C3, C4 or C5.
Here is a Quote from Vanderbilt School of Music:
“There are TWO conventions for numbering keys (notes) in MIDI. The most common is where MIDDLE C (note #60; $3C) is C3 (C in the 3rd octave). However, another convention was adopted by Yamaha Corp. for their synthesizer products which parallels the Octave Designation System used in Music Education formulated by the Acoustical Society of America. In that convention, Middle C is designated “C4”. The “C3 Convention” is the most commonly used octave designation system on standard MIDI keyboards…”
I have read elsewhere that middle C has THREE standards including C5, but always note number 60.