How to choose he zoom of the tiles ?

Hi all,
I have succeeded to generate the tiles by Mapnik.
My problem is that how to choose the size of the tiles ( I need to generate a tile that its zoom is 18 )
I have modified the file like this :
** render_tiles(bbox, mapfile, tile_dir, 0, 16, “World”)
minZoom = 14
maxZoom = 18**

the problem is that i don’t obtain tiles that have the zoom 18.

Can you help me please.

you are rendering from 0 to 16. replace 0 with 14 and 16 with 18.