I'm not really privy to sheet cakes so I'm a little lost about the amount of fondant ill need. So the cake measures 24"1/2 x 18" not accounting for the height on the sides so maybe more like 30"x24 is more accurate if I figure the height is around 3 inches or so since it's two sheet cakes stacked. How much fondant? Currently I have around 7 pounds of black fondant...will I need more? Thanks in advance! 

Honestly, I've never covered a rectangular sheet cake, but I have made a guitar and a 2D My Little Pony in similar sizes and I would guess that seven pounds would be enough.  I would certainly try rolling it out to the required 30" x 24" before I bought more.

might need more

https://www.cakecentral.com/forum/t/559313/how-much-fondant

I covered a 18 x 20 sheet cake. And your 7 lbs of fondant is more than enough. You will probably need 2 1/2-3 lbs max. There a beggar to cover. Not sure I’d be covering another sheet anytime soon.

idk -- earlene says she starts with 8 pounds for a two layer 14" square which is considerably smaller -- literally 7# may well be more than enough after it's all said & done but to get that monster on straight without much hang over would do me in -- God would close the ears of the angels for all the expletives i would 'share'-- i would want to roll out plenty so i didn't have to arm wrestle the fondant into place over the cake just so, 'cause i didn't have enough rolled out -- 

use any amount you see fit but roll it plenty big like 34x28 -- two inches extra all around --