Ok, not sure if I'll ask this question how it's in my head, but are pitching rates tied more to the fermentor volume or OG? In other words, similar to how a recipe can be scaled up or down, does a 2.5 gallon batch need half the pitch rate of a 5 gallon batch, all things being equal? or does a higher OG need a higher pitch rate regardless of fermentor volume? maybe another way, do we need more yeast because there's more room at the table (higher volume) or because there's more to eat (higher OG)? in typing this out, i think i answered my own question, but curious what others have to say

The pitch rate should be based on OG, the amount pitched is based on pitch rate and volume. Pitch rate is the number of cells per amount of sugar per unit of volume (million cells per milliliter per degree plato). To figure out how many cells to pitch you multiply the rate times the volume and times the OG. A 2.5 gallon batch of the same wort would have half the yeast pitched as a 5 gallon batch, but it is the same pitch rate. Bigger beers tend to need a higher pitch rate so there are more yeast to get through all the sugar. You need both, more yeast cells for the volume and more for the higher OG.

ok, i assumed that pitch rate and amount were interchangeable. that makes sense. so by increasing pitch rate, you'd necessarily have to increase pitch amount.

Without counting cells and testing for viability, you'll never know your exact pitch rate so we use slurry volume as a proxy. This is one of those variables I can't really control in my process so I resort to plan B: Do a 1.5 liter starter for every beer then RDWHAHB.

my reason behind this weird question is that i'll be taking a normal 5 or so gallon batch and splitting it into 5 buckets. It's going to be moderate beer, around 1050 OG i think. would i be ok (this being a relative term) to split one vial of liquid yeast amongst the 5 buckets (as evenly as i can)? or should i get 2?

For a 1.050 beer, I would pitch 150M - 200M cells (I tend to pitch heavy: 0.75 - 1.0 M/ml/P), so I would make a starter or buy two vials. You can split the yeast across the 5 one gallon batches, but swirl the yeast each time to make sure the yeast aren't all at the bottom of the vial or the last one will get more yeast than the first.