How pure does the protein need to be? It is very important that you attain the highest purity of protein. Any impurities in your protein reduce the chances of obtaining crystals.

What is the best possible concentration for the protein? Though there is no way of giving an accurate information on this question, 10-20 mg/ml has been a standard aim for most crystallographers.

What is the minimum amount of sample for screening against 96 conditions or one protein crystallization plate? It depends on the size of drop you want to use and also on the type of protein you are trying to crystallize. A minimum of 12ul and all the way to 20ul for a 100/100 size drop will be needed.