It all depends on the size of the files produced by the camera. This can be governed by whether you are shooting in RAW or JPEG, the pixel count of the camera and, to some extent, what aperture you are shooting at and even what ISO setting you are using.
The camera will give you a shot-remaining count when there is a card inserted. This is normally estimated on something like a 'worst-case scenario' basis (ooops, bit of management-speak there) and will most often be bettered in real life shooting.
Do you have a particular camera in mind? There are often guides to the estimated shot count per card on the manufacturer's websites.