XSLT » xsl:number » grouping-separator="{ character }"

The optional grouping-separator attribute is set to an Attribute Value Template or a character that dictates what character is to be used to separate groups of digits in the number being displayed in the output. White-space is permitted. The grouping-separator attribute is ignored if you do not also set the grouping-size attribute to the digit group size. The classic example is to use a comma to separate groups of three digits.