Close.
FWIW I'm glad to see you at least tried to work it out for yourself. There is lots of information on this site on how to build this sort of stuff if you are prepared to use the search button, read and learn. There is also a lot of information on RC forums too, a google search will reveal a lot of information on building packs from A123's including picture tutorials.
You need ~15s to get 48v
You need ~9p to get 20ah
That's 135 cells. At 10cells per pack you need 14X 36v Dewalts.
@ $140 a battery (conservative) that's $1960, not including supplies, charger, balancer.
But honestly, if you are asking these kinds of basic questions then you obviously dont know how to build the pack. Do yourself a favour, follow Fetchers advice and buy a prebuilt pack, that way it comes ready to use, wired correctly, has a charger and balancer if needed and will save you a whole lot of hassle and likely save you from very costly mistakes.
And this is no small project

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Honestly, once again buy yourself a premade pack and avoid a lot of heartache. For $2000+ you can buy an 5 series Xlyte that will far out-perform your golden motor, a premade batt to make it go and still be less than that figure. I'd fathom a guess that you are probably running the stock golden controller also which wont even really take advantage of what these batteries can offer.