It depends what you mean by many to many. In GraphDBs you can have several models of relations. In a graph DB, a node can point in any direction. Including to himself. You can also point N times to the same node using several edges. There are no limitations.
I think the one-one, one-many, many-one and many to many meanings in Graph Dbs are different from what we learn in relational DBs. We don’t have any issue to solve with many to many. Which is a solution for relational information in a DB that relies on Tables.
A “kind of many to many” relation, as it is done in relational dbs, can be mimic via “intermediary node”.
User -> intermediary -> User
This intermediary would be a “bridge” that creates a very handful and informative many to many relationship. The objective of the “intermediary” is to abstract information. But you do not solve the same issue as it does in a SQL for example, just abstracting information in intermediary nodes.