Unless you have an idiot tech and a machine from 1980, it is pretty obvious by 8 weeks if there is one or two in there.
It does happen. Mostly with twins that share a placenta, when the membrane between them is too thin too see. Then there is only one "blob" with two babies inside of it, which COULD be hard to see if you aren't looking thoroughly.
My u/s at 7 weeks with di/di twins (Different sacs different placentas), preeettttty obvs.