Given a balanced bipartite graph with $2n$ vertices. Do we have any upper bound for the maximum number of edges such that the graph is planar (i.e., a bound that is stronger than $6n-6$)?
In any bipartite graph, we can improve the upper bound. Combine Euler's formula $V-E+F=2$ with the inequality $2E \ge 4F$ caused by each face having at least $4$ sides (since there are no odd cycles) and we get $E \le 2V-4$. That is, with $2n$ vertices, we have at most $4n-4$ edges.
We can't improve the bound if the graph must be balanced. The graph below achieves the maximum possible number of $4n-4$ edges, is a balanced bipartite graph, and we can extend it to have $2n$ vertices for any even $n$.