At the time of writing, having a non constant expression for 'n' in window functions is not supported. This ticket proposes allowing the  'n' expression to reference the partition key (such that a given partition has a 'stable' value of 'n'), similar to how these expressions in $group can reference the group key.

Other alternatives:

• The 'n' expressions must be constants (and cannot reference any incoming document or the partition key), that is, keep the behavior which exists on classic at the time of writing.
• The 'n' expression can reference the values from incoming documents. Note that this is not well defined without a sort, as there is no guarantee around the order in which documents will be seen in DocumentSourceInternalSetWindowFields::doGetNext()