At what density (parts per thousand or per cent) does crystallization of sea salt start?

this depends on the purity and temperature of the water and also to some extent the availabilityof crystallization nuclei

roughly 36 grams of NaCl in 100ml of water at 25c.

 

this amount will increase or decrease depending on the water temperature.

YT is your figure for pure water or for sea water, which contains other salts?

I'm not sure the question is meaningful.

Seawater is saturated with, for example, sand. If you concentrate it slightly it will be supersaturated and silica will crystalise out.

 

If you are trying to get salt to crystalise then YT's answer must be pretty close, even allowing for other things in the water.

i was thinking of the common ion and uncommon ion effects

I guessed that, but sea salt is still pretty much salt. The other components are in relatively small amounts, next on the list of elements in seawater is magnesium, and there's more than 10 times as much NaCl as Mg. Strictly we ought to worry abot the temperature and the particular source of teh water, but "About 36%" is probably as good an answer as any.