XSLT » xsl:number » grouping-size="{ number }"

The optional grouping-size attribute is set to an Attribute Value Template or a number that dictates how many digits are in the groups that are being separated by the character specified in the grouping-separator attribute. The grouping-size attribute is ignored if you do not also use the grouping-separator attribute to specify a separator. The classic example are groups of three digits separated by a comma.