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+1 on them picking out the weaker ones. My local fish guy told me when I first started that if I wanted 3 buy 7 and within a year I will have my 3. The fact that he was wrong and I had 7, 2 years later has no bearing other than me teasing him... by the time year 3 rolled around I actually was at 3 LOL.
To answer your question I would suggest exactly what he told me. Buy 2 times what you actually want and figured at full grown 15-20g per chromis. This is because they never stop moving during the day and need room to school.