In total Billie bakes 3 x 11 = <<3*11=33>>33 pies.
After Tiffany eats the pies, this many pies are in the refrigerator: 33 - 4 = <<33-4=29>>29 pies.
She needs to buy this many cans of whipped cream 29 x 2 = <<29*2=58>>58 cans.
#### 58