It was his pope name.
His birth name was Felice Peretti.
Sadly, there does not seem to have been a Pope Sixtus the Sixth.
Sixtus I - 117-126 AD
Sixtus II - 257-258 AD
Sixtus III - 432-440 AD
Sixtus IV - 1471-1484 AD
Sixtus V - 1585-1590 AD
(Yes, I know it is redundant to include AD in Papal terms but I was just making it clear they were years.)