This is exactly one reason you make a gauge swatch.
Here is what you can do:
1. Make a gauge swatch with the needles and yarn that you have available.
2. Count the number of stitches for four inches or ten centimeters.
3. Divide that number by four. This will give you the number of stitches per inch.
4. If you are making a hat multiply the number you have for step 3 by the circumference of the head.
5. Then you will have the number of stitches you must cast on.
This is why it is very important to do a gauge swatch. For practically every project I create a gauge swatch. With that swatch I can adjust almost any aspect of the project or garment to fit my custom needs.