hello @Nicole , welcome to the community,
I noticed I responded to a question that resembled a lot your question, but your is also about a single item. I use ‘ListName’ to reference the A,B, C, D etc.
If(ListName.Count()=1,
ListName,
Format("{1} and {2}",
ListName.Slice(1,Lamb.Count()-1).List(),
ListName.Last()))
to understand the logic behind it a bit better:
Cheers, Christiaan