Using harvested yeast

Im planning on harvesting and washing my yeast from my current brew. My question is once I have my jars of washed yeast, how do I know how much of that yeast to pitch for my next brew? Do i throw it all in or do i have to measure it some how? Any and all input greatly appreciated!

Check out mrmalty.com under yeast tools. Select the 'repitching from slurry' tab. You can enter the gravity and volume of your planned beer along with the harvest date of your yeast and it will calculate the volume of slurry required. There are other calculators out there too, but I haven't tried them.
As a (very) rough guide, if you're repitching within a couple of days into a similar beer to the one you harvested from you'll need about a quarter of the yeast produced from the first beer.

Mr. Malty says 4.5 billion cells per ml of dense slurry.
I figure 50% = 2.25 billion / ml (if it's been sitting in the fridge awhile), and even round that down to 2, so if I need 100 B cells, I pitch 50 ml of dense slurry.

I have washed yeast for a couple of years now and I always make a starter. I have pitched slurry directly from one fermentation to the next without washing and used Mr. Malty numbers to measure it out. You can save unwashed slurry in the fridge for about a week if you don't have a fresh batch to pitch right into. If it will be more than a week make a starter to build up one jar of your washed yeast.

Thanks guys for your replies, i do appreciate it. Sorry for the rookie questions, i really want to make sure i do this right. So is it necessary to wash slurry? When saving yeast, how many times should I let the yeast set and collect before i do my final collection. Also how much liquid shod be in with the yeast compared to the amiunt of yeast?

You do not have to wash if you are going to pitch from one batch immediately (or soon) into another. You can scoop from your bucket with a sanitized spoon into a sanitized container (like a small Pyrex measuring cup or a Mason jar) and cover with sanitized foil and keep it in the fridge. If you wash then follow the yeast washing sticky here on HBT. The latest Chop & Brew episode (3?) on you tube (glad to have you back Chip!) actually has a really good video demonstration from a reputable source.