A right angle is equal to 90 degrees. The standard piano has 88 keys (52 white keys and 36 black keys).
Thus, the number of degrees in a right angle (90) minus the number of piano keys (88) is 2.
The above mentioned figures are part of the 24 "equations" by Will Shortz, printed in the May-June 1981 issue of Games magazine, with an acknowledgment to Morgan Worthy. Many people have now added to the list of equations. See them here.
Of interest, in the late 1880s, popular piano manufacturer Steinway created the 88-key piano that is the standard today. Other manufacturers followed Steinway's lead and 88 keys has been the standard ever since.
The 88-key piano features a full seven octaves, plus a few other notes. Why stop at 88 keys? Most composers don't write music that includes notes beyond those available on the 88-key piano. Plus, notes lower or higher than those on the 88-key piano aren't easy for the human ear to hear as distinct notes beyond those that already exist. See Wonderopolis.org's Why Do Pianos Have 88 Keys? for more info.
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