AMD Mantle promises a lot of draw calls per frame. In a tweet (see this article), Jonhn Carmack said that OpenGL can give similar improvements via a correct use of some extensions.
Cass Everitt and John McDonald (both from NVIDIA) have published an interesting set of slides about how to use modern OpenGL to reduce graphics driver overhead and increase the draw call count and unveiled a bit what Carmack had in mind…
You can download the PDF here:
In short, the combined use of GL_ARB_buffer_storage + Bindless / sparse textures + shared UBOs (uniform buffer object) + GL_ARB_multi_draw_indirect can help to increase the draw call count…
The original slides can be found HERE.