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.
The weight of your yarn is a major factor in how many chains you'll need. Heavier yarn will make larger chains, so you will need fewer. If you are selecting a lightweight yarn, on the other hand, be prepared to increase your number of chains.
Some stitch patterns also require a multiple of a certain number of chains, something which you will also have to remember when creating your foundation row.
The guide below can be a great help in working out roughly how many stitches you will require - but remember to factor in things like yarn weight, pattern repeat multiples, hook size and your own personal crochet style.
If you have a disaster, and your blanket ends up not being the right size for your bed or intended project, you can always add a border, as I did myself here:
Alice Cullerne Bown
I used a granny ripple stitch for the blanket but then had some fast thinking to do as it became clear that I had accidentally made a single bed size instead of a double. The chevron shape of the stitch used up much more of the foundation chain than I was anticipating. It was much too late to unravel the project, so I improvised and devised a granny ripple border along the sides which extended the blanket by several inches. No one has ever asked me if it was meant to turn out that way and I've had many compliments on the final result. A nice border can be your friend!
Happy blanket making.