<pb/> elements can mark a page break in different positions in the TEI structure.

//pb/position() gives me the positional number of all of the page breaks, 1 thru n.

and something like (//pb)[position()=3] gives me the third page, for example.

But I want to process the document thru the identity transform, with only a different template for the page breaks.
And I need, in the context of that template, to get the page number in the sequence.

You can use
  <xsl:variable name="pos" as="xs:integer">
    <xsl:number level="any"/>
  </xsl:variable>