I have a case where I know that a person had 4 marriages, one after each other. I also know the other persons. But I have no information of the specific dates. I do have the order of marriages relative to the person. How can I annotate that?
Like Marriage number 1, number 2…
In the Relationships view there is the Change Order of Parents and Families icon 
on the toolbar and next to the Parents and/or Families when there is more than one. I always sort oldest to newest so the most recent is the last entry. Just be consistent in your sort.
Even if you do not have the marriage date you can still add the Marriage event and use the Description field to number the marriages. I use the Description field to add the wife’s previous married name; Mrs Mary Smith.