For which language do you ask?
I only translate into Welsh / cy.
I am not familiar with Welsh and don't know off hand, if there are strict limitations for Welsh in the system.
Have you tried {{PLURAL:$1|1=diwrnod|2=deuddydd|3=tridiau|dirnod}}. See Plural for more information.
{{PLURAL:$1|1=diwrnod|2=deuddydd|3=tridiau|dirnod}}