vincenzo,
Most city locations can support the 'full' 20 pop (or more), but not all.
When you get to Railroads you can get 4 food for an irrigated grassland, 3 for irrigated plains, and 2 for irrigated desert. Only Hills and Mountains will not ever support 1 pop per tile.
This means that early on in very hilly/mountainous locations you can not support a full city, but since size is limited to 12 until Sanitation this is not normally a problem, as rails and hospitals appear around the same time.
Often, near a mountain range you can place cities around the outside so that they can all use a few mountain tiles each and share the low-food tiles.
Another mitigating factor is that it is good practice to have less than 21 tiles per city. Overlapping cities can provide several benifits, including the sharing of low food tiles.
Edit: Note that before Sanitation and Rails it can be a slow process to grow your cities to size 12 if you have low-yield tiles. In this case I will often sacrifice having *any* hills used in favour of using the irrigated grass/plains to get growth as high as possible. Sometimes I have cities stranded at size 3 or 4 because they have only plains and no fresh water, and just 1 decent tile in the radius, these cities are only founded as long term investments, or as resource-grabbers, or for strategic placement value, it takes a while for it to pay off sometimes...