Why is it restricted to 2 bins?
Since it is synchronous, it is expected to quickly return an answer. And because the calculation time can increase sensitively when multiple bins are used, we decided to focus only on the use case of optimizing load for a truck that may have an additional trailer (thus 2 bins), for the sync operation.
How to get rid of this obstacle if you need to plan more than 2 bins?
If more bins are needed, then it is possible to use the asynchronous operations. More detailed information can be found here