Making like 5k cubes, into one mesh?

I heard this is called chunking, I need it to make my game really smooth.

Check out After playing minecraft….

Also look at Voxel Resources & Engines for Unity

You may or may not wish to instead make a Mesh that you can modify rather than trying to combine a large number of Cubes, especially if all of these Cubes feature the same texture.

Unless you’re trying to replicate Minecraft, I wouldn’t see much of a need to use too many of these custom Meshes.