Let's suppose, you have a number in A1 and you want to get the column Name for that.

Hence, if A1=1, you want "A"
Hence, if A1 =26, you want "Z"
Hence, if A1=27, you want "AA" and so on.

The formula to derive the column name would be be -

=SUBSTITUTE(ADDRESS(1,A1,4),1,"")