How much volume of starter to keep around

Easiest way to reduce discard - don't make any. I don't have any use for it, so I don't make it, simple. Enjoy!

I have a no discard starter regimen.  At the end of the week I try to have about 5 g or so of starter left.  If I have more than that, I smear the excess on a Silpat and allow it to dry and add it to the freezer bag of dried starter I have as a back up.  I look at what I might need for the week and then feed my starter so I have enough for the week’s baking and aim to have only 5 g left after all the starter needed for the baking.  So this week I knew I needed about 30 g of starter to make levains.  So I did a 1:3:3 feeding - 5 g starter, 15 g water and 15 g whole rye flour to give a total of 35 g.  After I make my two levains needing 14 g of starter I should have approximately 5 g left.  I do this each week.  If I happen to need more starter than I have, I can do a two stage levain build to get enough or feed more starter a bit sooner than usual.

So simple to have zero waste/discard.

Benny

but there is a small amount every time I feed my starter. (I also keep both WW and Rye starters and I treat them the same)  I always feed 1:3:3 s:f:w, but in answer to your question:  I keep 35 grams total of starter, mostly in the fridge.  I feed 5 grams of starter with 15 grams of flour and 15 grams of water, and save the 30 grams of not-discarded balance.  I keep the not-discard in the fridge too, in a small 2 cup Tupperware container.  When I have enough saved "discard", we make sourdough pancakes and/or waffles.  Sometimes I have to make up a batch of starter just for pancakes since it takes a while to accumulate enough discard, and we don't want to wait. :)

I haven't had any issue with starter vitality keeping such a small amount, but I do keep backups, both dry and frozen.  Because I don't bake even weekly much of the time I keep my starter in the fridge, so I am only feeding it when getting it ready to bake.  I'll be baking next week so Sunday night I'll take my starter out and begin getting it ready.  It's always a little sluggish at first, so I'll feed it twice a day for 2-3 days, until it gets nice and active, then I'll feed it once more to pull for my levain.  Then it goes back in the fridge until I need it again.  I'm guessing this is similar to what you are doing too.

My $.02 worth
Best of luck
OldWoodenSpoon

of ways not to have discard. 

I keep about 1/2 a cup in the fridge. The night before baking, I take out 40-50 g for 12 loaves of bread. That I feed in builds until I have enough for my dough. The starter gets about 1/8 cup water and enough wholegrain flour to make a thick paste. Stir well into the starter and I put it directly back into the fridge. No counter time at all. I bake once a week and this provides enough food for the starter to live in for the week. It really needs to be fed once a week. I learned the hard way not to neglect my starter.