Building on AnnaAG's answer of 21 40 gallon barrels.

I think you can get 22 barrels in.

I'm making the case that you could get 4 barrels in 1 layer on their side.

There is some overlap in the image but I think that can be ignored because of

• the curvature of the barrels
• the barrels actually being 8.8 squares high not 9 like in the image

If you allow that then you can stack the barrels in the following way:

• 2 layers of 7 upright barrels which is 176 cm
• you would then have 128 cm left at the top which is enough for 2 sideways barrels on top of each other (120 cm)
• which means you can have 2 layers of 4 sideways barrels
• for a grand total of 27+24=222*7+2*4=22 barrels, an increase of 1 barrel!