i only have one 10g pack of dried yeast, but want to brew 10 gallons.Can you make a starter with dried yeast? Is it benificial to the yeast or should I just rehydrate?
Most dry yeast packs are intended for 5 gallons at 1.040 SG.
You need to basically add a pack for each .020 points above 1.040 SG x volume / 5g
In your case I would just grow more yeast with a starter. How big of one depends on: Yeast Package date, wort SG, and volume. All are critical to calc the proper pitch.
For example you need about 280b cells for 10gallons of 1.040 wort. That's 15grams of dry yeast less than a month old. Or a 5qt starter from one liquid vial less than a month old.
When making a starter for cell growth, you want it to ferment completly, cold crash it and decant the starter beer off. Just pitch the biomass "yeast trub".