Having the default item ID from SharePoint can be a little bit tricky – if you erase an item, you lose the sequential order, or if you are getting the items from a number of lists you will have duplicated ID’s.
So, if the ID is not important, and you only need to number it, you can use the position (add +1 so it will start from 1 instead of 0), For example (in the rowview):
But, if the order should be descending you can use the number of rows and the position. For example:
In the call to rowview:
<xsl:with-param name="Rows" select="$Rows" />
In the rowview:
<xsl:param name="Rows" />
<xsl:value-of select="count($Rows) - position() +1"/>
It will even work with paging (will preserve the number from previous page and not start over from the current page).