well... the straight answer is:

you put 20g NaCl per 100mL of media...

Now... what exactly are you preparing? high salinity nutrient agar? that would tipically be around 3% NaCl...

If you grow plants and not microbes, that concentration can be used, sometimes... Its just weird you're looking for so much salt in the culture media... 

From my understanding you would add 200 grams of salt per 1L of water.  

And to answer Rafael's question about why you would use such a high salt concentration it would be optimal for halophilic microbes.   

Which hopefully is what you are trying to culture.  

Best,

Tyler Gladwin

Both anwsers are right:

20g NaCl per 100mL is the same as 200 grams of salt per 1L.

There is nothing weird in this amount if you are interested on extreme halophiles as Tyler sayd.

Note I left intentionally NaCl (from Rafael answer's) and salt (from Tyler's).

For cultivation purposes you don't need ultrapure NaCl, so common salt (NaCl with some impurities) is cheaper and can even help for better recoveries/growth.

You can adjust the pH after adding salt.