for calculating the number of insulator disks needed for creepage is line to line voltage or phase voltage used?
From comments: -
What does the insulator connect to on both its ends? If it is suspended from a pylon then it will be ground on one end but there's no clue in your question. Only you know this part of the puzzle.

My apologies, I thought that was obvious when I mentioned that its a
transmission line. Yes it is insulating the tower (ground) from the
transmission line.

In that case you should consider the worst case scenario phase voltage that might be seen. The normal case is just the phase voltage (as per line voltage divided by \$\sqrt3\$) but, if there is a phase lost during distribution and the sourcing transformer is a delta output (no neutral) then there is a possibility that the insulator might see somewhere between phase voltage and line voltage.
You should also consider that lightning strikes are quite common on pylons and this will make an impact so, is there a top earth wire running from pylon-to-pylon that can partially umbrella protect the lines? These are things you might need to consider as per this: -

Picture from this answer