Many people would like a flat answer to the question of how many chains you need in your foundation row to create a blanket or afghan, but there are a lot of variables to consider which could make all the difference to the way your final project turns out.
Much depends on how loosely or tightly formed your own crochet stitches tend to be. Only you will know the answer, but if your chains are usually tightly bunched, you might consider using a larger hook. If they tend to be large and open, you might need to go one size smaller.
If you do change hook size, bear in mind that you need fewer chains with a larger size and more chains with a smaller one.