need help going round in loops

Hi im working on a project at the minute, where im creating a template book in excell.
I can add pages and rename them to Page 1 , Page 2 etc.. i can insert new pages inbetweeen the count and not screw it up. how ever in P20 on every page i will have a time . On the frontpage (sheet 1 ) i would like to display this TOTAL time in cell I16.

i know if i manually enter the formula =sum('*'!P20) it will perform. but it only does this one time. Ive recorded a macro added it to a button and can only get a single use from it.

If somebody could help me right a VBA code to first count how many worksheets are present and then add up all P20's and then display them on Sheet1 I16

If this could be made onto a button so the user can just click it to perform i would be most gratefull :)