"They visited us only once."

Translation: Navštívili nás jen jednou.

[alturing]

Would "jenom" instead of "jen" really be wrong here?

[Paul440375]

It's correct as well. "jenom" and "jen" are interchangeable in every case. Maybe "jen" sounds a little, but just a little, more formal, but that's it. Both are equally used in spoken and written language.