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Is this a question of currently being married and marrying someone at the same time as the current one, or marrying after the ending of a different marriage? If it is a polygamous relationship I would follow the above information but if one is just asking about a new marriage, you would use traditional rules of a relationship chart without involving such complexity. Or, if perhaps if one is comparing the current spouse to a new potential partner.

The same is true for any subject. For comparison, when one is looking for a new job they don't go to great lengths to analyze the current job and the new job. How many jobs does one have in their life and why does it matter when asking a horary? The only time I would compare is if it involved a move/great change or decision between two, in which case I would use a "stay or go" chart where the ascendant is the current situation and the 7th is the new one (and the derived houses represent the areas of life).

If this is simply a question of how many marriages/qualities of the marriage, many traditional authors have rules you can use. The natal chart is usually used for this but horary could be also. Lilly stated the following in Christian Astrology:
Behold from the degree of the tenth house to the degree of Mars, and so many Planets as you shall find between them, so many Husbands shall she have;
But if Mars be in the eleventh house, then look from Mars to Jupiter, and judge accordingly: some judge from Mars to the Lord of the tenth;
These rules are Arabicall: plurality of Husbands is best adjudged from the Lord of the seventh and Sun, Mars being in common Signes, or many Planets in the seventh, or Sun in sextile or trine to many Planets in the seventh, argues plurality, or more than one.
Usually for males Venus and Moon are more important. Keep in mind the traditionalists didn't analyze gay relationships and any text on the matter are offensive to read.

In my experience, these rules don't always work that well. They might show potential for marriage(s) but I believe people ultimately have choices. With horary it's generally more useful to ask about something significant in your life in the current or near future (and sometimes the past).