Answer by Michael for How to get the number of Pages in a single Post Pagination?

have a look at https://codex.wordpress.org/Global_Variables#Inside_the_Loop_variables

within the loop, the global variables related to the <!--nextpage--> tag are:

$page - the actual sub page,

$multipage - (boolean) =1 if nextpage was used

$numpages - total number of sub pages