A simple and useful addition to xsl:number would be an attribute

start-at="expression"

to control the first number in the numbering sequence (defaulting to 1).

Useful for example where numbering is to run across the documents in a collection: 

<xsl:iterate select="collection('chapters')">
  <xsl:param name="first-fig" select="1"/>
  <xsl:apply-templates>
    <xsl:with-param name="first-fig" select="$first-fig" tunnel="yes"/>
  </xsl:apply-templates>
  ...
  <xsl:next-iteration>
   <xsl:param name="first-fig" select="$first-fig + count(.//figure)"/>
  </xsl:next-iteration>
</xsl:iterate>

<xsl:template match="figure">
  <xsl:param name="first-fig" tunnel="yes"/>
  Figure <xsl:number level="any" start-at="$first-fig"/>
</xsl:template>

Requirement extrapolated from a question on xsl-list today.