RIS half batch, half yeast?

So I'm wanting to do a RIS but only 2 gallons. Of all the forums and recipes I've read almost all of them call for 2 packs of yeast for 5 gallons. So my question is, since I'm only doing a half batch ( or there about ), do I only need to use one pack of yeast? Or is there something I'm not aware of with high gravity and the amount of yeast?

It depends on the OG but more than likely one packet will be fine. Check mrmalty.com to be sure.

yes, as far as i know thats the case.

if someone is making say 10gallons of beer, they'd need twice as much of a yeast culture as 5 gallons. i thin for a high gravity beer however, you also want to make sure your yeast is healthy and kind of build it up towards tackling high gravity wort with a starter of some kind

What's your anticipated OG? Are planning on using a single pack of dry or liquid yeast?

If you're under 1.085 you could probably get away with a single pack of rehydrated dry yeast for a 2 gal batch. If you're using liquid yeast I would recommend doing a small starter. I do mostly 2.5 gallon batches and on some of my higher gravity belgiums I've done quart-2 quart starters (decanting before adding).

The general rule is that the higher the gravity the greater the number of healthy yeast cells are required for good fermentation. Mr. Malty is a good place to check pitching rates and to make sure you have adequate yeast.

I made a half batch (2.5 gallons) of a belgian specialty ale last weekend and according to mrmalty.com i still needed to pitch a full vial of white labs yeast.

thanks for all of the quick replies. I will check out mrmalty. My OG is expected from my calcs between 1.110 and 1.120