Current capacities (these add up to more than the total so presumably they're number of seats, including those we don't or can't sell) ^denotes approximate
Kop 10,221
Laver Stand* 7,500^
John Street 6,800^
Bramall Lane End 5,680
Bramall Lane Corner 1,900^
Terrible Corner 900
Presume you're talking about United adding a new tier to the existing Laver Stand and keeping much of the existing space open, much like Liverpool did at Anfield? The plans as currently submitted (and you can guarantee that they'd be redone) add 5,400 to the capacity with the second tier and corporate space.
FWIW I fully agree on us being at capacity but only when we're either flying in the Championship or not being abysmal in the Premier League.