I'm sorry if this is long winded. I don't want to be a pretentious git, but in the interest of teaching a man to fish and giving the benefit of experience I'm going to waffle ... sorry!
Yeast slurry depending on thickness can quite happily be estimated at 1-3 billion cells per ml. Years of doing cell counts confirms this for me. Typically good slurry is almost always 2 billion cells per ml, but your mileage (process) and viability might vary. Slurry is what you get when you crash/chill the starter wort and off decant most of the clear portion at the top, leaving behind the solid layer at the bottom and then swirl it up with the little liquid that remains. If you have at least 40-60% solids before you swill it up you can quite happily (depending on strain) estimate it at 1 billion cells per ml.
This is handy, because standard pitch rates for ale is 1 million cells per ml per degree plato. A billion is a thousand millions and a litre is a thousand millilitres therefore 1 billion cells will result in 1 million cells in a litre. So basically you take the amount in litres, times it by the degrees plato and use that many ml of slurry consisting of 40-60% solids estimated at 1 billion cells per ml. To convert specific gravity/starting gravity into rough plato divide it by 4. So 1.040 becomes 10 degrees plato.
If your 10 litres of wort is 1.040 you'd need 100ml of slurry.
If 100ml seems like a lot bear in mind I use commercial pitch rates and like a 72 hour primary ferment. You'll likely get away with less, but you'll have a longer ferment, aeration becomes import and crucial if you intend to harvest yeast. There are so many fudge factors in this, but it'll work out fine 99% of the time. Your slurry may have more or less cells, but your viability might be lower or higher than expected. Your pitch might be off, but your volumes might be off. 1 million cells per ml per degree plato is a starting off point and a generous one at that. I've had fermentations complete fine with a quarter of that, it contains a healthy margin of error.