There are many causes to this. The number of lights you have don't matter, it's mainly how much area is covered. That is, the total area of all lit surfaces must not exceed a certain value. If you use a low scale for your blocks, such as 0.2, it has a big impact. It may seem like a 256x256 surface, but with a scale of 0.2 on each side of that surface, it's actually using the equivelent of 1280x1280 if unscaled. When the total area exceeds a certain value (of which I don't know what it is - 16 megapixels?), you'll get this error. There are numerous fixes, each with their own pros and cons:

1. Break up your level into a few parts and pieces and build them as a map entity replacing the areas in focus. A shortcut is to save the level under another name, delete what shouldn't be in your section, then build this as a map entity.
2. Replace some areas with models instead.
3. Increase the texture scale on large surfaces, if possible.
4. Use the "flat" or "none" flag for surfaces that are never seen such as from junctions of walls and "flat" if in entirely unlit areas. Flat-flagged surfaces don't get lightmaps and can help.
5. If nothing else works, then try deleting some parts of the level.

This is all I can come up with.