P₁ V₁ / T₁  = P₂ V₂ / T₂

20 * 2.82 / 300 = 1 * V₂ / 273

V₂ = Volume of gas at STP = 51.32 l

 Volume of gas filled in the balloons = 51.32 - 2.82 = 48.50 l (because gsa equal to the vol of cylinder would still remain in the cylinder as we cannot expect a vaccum).

Now divide this volume with the volume of sphere (4/3 r³ ). You will get the number of balloons filled.