Just one lightmap per level

I've tried for some while now to find the answer to this problem, but I've yet to find the information I require.

I want just one single lightmap per level independent of how many objects I have. But Unity apparently chooses for me exactly how many I need, and that's the issue. I would rather have the quality be worse with one lightmap rather than having several.

So my question is how would I get unity to understand I want one lightmap for the whole scene?

currently unity gives me two lightmaps for a small scene.

This is for single lightmap baking.