You find the median, or 50th percentile, of a group of numbers by sorting them and, if the quantity of numbers is odd, taking the middle value in the list and if even, the midpoint between the two middle ones.  Once you have the median, you can say that half of the values in the list are greater than this median and half are less.

A list of one number is an odd-lengthed list.  So the middle number is the same as the only number:54 (in your example).  If the only value you have is 54, it does not matter how many of them you have; the median would be the same.

However, on a practical level, the median of a list of length one or a list of numbers all of which are equal is meaningless.  So, from this practical standpoint, the answer to your question is "no," there is no useful definition of "median of just one number."  You cannot say that half of the values are greater then this number and half are less.