Mirepoix is the magical base of many sauces, soups and stews.  When asked for mirepoix, or merely a combination of onions, celery and carrots, the following percentages apply:

50% onion

25% carrot

25% celery

The nice thing about this, is if you don't have a lot of celery, you can beef up the carrots a bit and vice versa.  But the basic percentages apply, you'll always want to have a bit more onion.