i can fit 6 more cells, why shouldn't I?

48 cells gets me 12s4p but I could fit 54 cells and I have them. Whats the story of the 12s4.5p?

You shouldn't. I believe a 13S-4P will have more useable watt-hours (WH+.

Think of this as a 6S-4P and a 6S-5P, two batteries in series. The 4P battery will run down faster and hit LVC sooner. A rough estimate says it will last a little longer than two 6S-4P's, but you're not using the extra 20% WH in the 5SP battery when the 4S is used up.

Now if you have the above two strings combined as a single battery, you have to recharge. It's bad news for any BMS. The battery will get unbalanced on the first use and it will take extra long for a balance BMS to fix that.

ok thanks. was think of 12s4p and 48 cells vs 10s5p and 50 cells and if there are any variables I'm missing. 5% more watthours and probably safer for the esc components vs a bit bigger current draw. Maybe in the long run the 12s cells would last longer. its spitting hairs and they both work but wondering while I wait for last parts

10S-5P vs 12S-4P? You have enough cells for a 13S-4P, I like that better if this is for an ebike. More S means faster motor speed, but it depends on your riding style. Sometimes slower is better. I think range is about the same though.

Having run both 10S and 13S on the same ebikes, I think the response feels a bit better on the 13S vs 10S when both are almost used up. The 13S will be down to 42V and while it will not accelerate anything like a fresh 10S at 42V, I think it is still more lively than when the 10S is down to 30V.

Why dont you find one more cell and run 11s x 5p = 55 cells, 46 v..?

du. don't know why 11 got skipped. good idea! I'm too used to lipos coming in packs of 2