The usual way to record stillborn children is with inherited name only, and identical birth and death dates.
The inherited name is derived from the record in the same as is it for other children; if not explicitly stated, it is typically the last name of the father, the last name of the mother, a patronym or a matronym.
If the gender is documented, it should be recorded. In the rare cases that a stillborn child has been given a name, you must use it.
You may wish all parents named their stillborn children, but it is not hard to understand that many would rather not do so.
Lack of a given name of is a historical fact you should respect.
You should not make up names, use a phrase like "baby", "stillborn", "unnamed" or anything like that as a name, nor use numbers. Your software will "name" and number all children when you create a report, according to the conventions for that report.
Software that does not allow an empty given name field fails as genealogy software.
Use of the abbreviation N.N. (Nomen Nescio, Latin: name, I don't know) may seem appropriate, but N.N. should not be used in lieu of a given name.
You can use P.N. (Prenom Nescio, Latin: given name, I don't know) as a temporary workaround for the limitation, but the real solution is to upgrade to something better.
In case of stillborn multiples, the children should be entered just as they would be entered if they weren't stillborn; in order of record.
Identically named and unnamed children of the same parents are a fact of genealogy. Their different birth dates, death dates, and the different records these dates are based on, are how you tell them apart from each other.