When the answer to "Is this the first marriage" is yes, leave the number of marriage field BLANK by simply skipping over it with the TAB key.
When the answer is no, then type EXACTLY what is written on the document. If it says "once," then you type "once." If it says "two,' then you type "two." And if it says absolutely nothing, then you type absolutely nothing, and skip the field with the TAB key.