You will have to find the best number of turns by experimentation.
The construction of the coil will make most difference to sensitivity, you want it to be fairly wide diameter, I would guess around 15cm would be a good starting point. If you make it smaller it will be more sensitive but it will only cover a small area. Making it bigger will increase coverage but make it less sensitive.
I would start with 50 turns of coated wire (as used in transformers) and mount it so it is protected from being touched but not close to any other metal items. For example, fixed to a plastic sheet. Again there is a compromise, fewer turns will make it oscillate at higher frequency which might make the frequency shift harder to detect. Adding more turns will make it more dependent on the coil itself than the metal you are trying to find.
Personally, I would change the two capacitors across the coil to 1nF and experiment with the coil to aim for a frequency of around 20KHz to 30KHz.
Brian.