Culturing yeast?

So, today is the first day I 'washed' my yeast. (*Round of applause?!*)

Quick background on my process. Boiled 500ml of water, and chilled to 75 F. Pitched into my fermenter, gently sloshed around. and used a funnel with a screen to help keep out the trub. I cooled the slurry, poured off the top 'wort', used a tall pilsner (stanatized of course) glass and carefully put the yeast in there temporarily, cleaned out the flask, then put the yeast back in.. Repeated this several times. Now, to prove to myself that what I obtained was yeast I prepared a 1600ml starter, (just for kicks), and pitched the yeast... I was successful as I am getting 'fermentation', but now I am interested in just keeping this strain in the fridge. I was looking at glass test tubes with screw tops where I could put 90%+ yeast slurry... But don't quite understand how to calculate a more precise amount of yeast cells per ml. NB has 20ml test tubes that would work perfect, but from what I can figure out online it seems I would need say, 45-50ml of yeast for a 1080 OG brew. I would like to know how many cells I would have in each tube, that way I could put in, say 100 billion, more accurately and generate a new starter to pitch.. instead of making too small of a starter or too large. I feel like there isn't a very precise method without a lab, but maybe someone has figured it out?

Also, when it says the yeast can only be used 10 times, is it 10 times from the original strain? or can that be considered exponential? i.e. washed yeast turns into 5 - 100billion viles, where each of those creates 5 - 100 billion viles, so on and so forth. At the end of the day you could buy one pack and use it for 1 > 5 > 25 > 125 > 625 > ....>.... you get the point, brews! At some point you are going to run out of test tubes... At what point are the yeast going to tell you to F off and they die? or atleast start creating off flavors or inconsistency.

You can purchase cheap 50ml screw top plastic tubes that have measurement scale on them. These are just slightly larger in volume than the WhiteLabs vials which can hold 45ml. This is what I currently use and find excellent - you could even put them in the pressure cooker if you wanted.
I put about 30 - 35ml of rinsed yeast in each and then I know what I am starting with when I go to make a starter.

If you store vials of rinsed yeast from an initial starter then conceivably you could go on for a long time without having to buy another pack/vial.
As to how many times you can grow it up in a starter before it mutates enough to notice a difference is open to debate and affected by quite a few factors. It is easy to start creating too large a store of rinsed yeast for your needs and end up dumping them because they get too old.

If you are going this far, read this thread: https://www.homebrewtalk.com/entries/freezing-yeast.html

I haven't tried it yet, but the concept is extremely intriguing. Freezing in pitchable quantities could save lots of $$$.

Ale yeast's can go for 10 or more generations and still be good. But i wouldnt go more than that for a lagering yeast, they tend to get lazy.