You can usually find answers to very typical questions like this
in the HELP CENTER:
On the "Person/Details" page/screen for the individual/person, go down to the "Family Members" Section, there you will see the first 'Heading', on the right-hand-side, which is, "Spouses and Children"; and, just below the word 'Children' you will see an "+ Add Spouse" what is a 'Button'/'Link', select that.
You will then be presented with a "Pop-Up" (aka: "Modal") 'Window' to allow you to add, ANOTHER. an ADDITIONAL, "Spouse'.
You can add as many "Spouses" (ie. Couple Relationships) to an individual/person that are required - there is no limit.
On top of what has already been proffered by 'FamilyBible', here is another "Knowledge Article" in "FamilySearch" on the matter:
How do I add a spouse or partner in Family Tree?
I hope this helps.