It really does depend on a lot of factors.
For example, a developer who is working on three items at the same time that touch on completely different parts of the code base may find that they lose efficiency due to task switching.
Another example might be a developer is working on an item which requires some slow, hands-off builds to take place. During the time the builds are taking place they will have little to do. In this situation you can imagine that working on at least two items would be more efficient.
This is why Kanban takes an experimental approach. We set the work in progress limit and then see what kind of flow rate we get. Then we tweak the limits until we maximise efficiency.
My recommendation would be to start with a low work in progress limit, measure the flow and then slowly increase the limit while constantly monitoring the impact of the change.