If you are going to sometimes pass a count and other times not, you should set up separate keys for them:
counted_user:
one: User
other: Users
user: User
t("activerecord.models.counted_user", count: 1)
t("activerecord.models.counted_user", count: 2)
t("activerecord.models.user")
If the default is only used in minimal circumstances that don't warrant a separate key, just call it directly:
t("activerecord.models.user.one")