Oh Welcome to the forum Ted!
So, first thing you'll want to identify the battery voltage you need, each 18650 cell is normally nominal 3.7V. You can connect cells in series and increase voltage (series of two cells is a 2S 7.4V, series of three is a 3S 11.1V and so on). You can also connect them up in parallel to increase current (for two 2000mAh cells you'll make a 2P = 4000mAh @3.7V, three cells would make 6000mAh @3.7V and so on). After these basic calculations you should be able to discover how many series and parallel packs you'll need.
For the BMS, instead, you'll look at how much charge and discharge current you want to manage for this battery (discharge is more critical because you can decide your charging current but discharging current could vary with the load). Consider that both charge and discharge currents will divide more-or-less equally between all the cells you connect in the battery. And you must respect the maximum discharge current of each single cell bacause you don't wont to pull current exceeding cell's specifications. My 25$ dumb Daly BMSs (dumb = no bluetooth, no fancy stuff) are rated 20/40A (20A charge, 40A discharge).
Enjoy!
