How deep for a barrel pond?

Thinking of having a barrel pond.It will stand on the patio and just contain plants.
I'm thinking it does not need to be too deep.Maybe something like this?
https://www.amazon.co.uk/Stewart-Blenheim-Barrel-Copper-Effect/dp/B0194IQGNS/ref=pd_bxgy_86_img_3/260-6684593-2651665?_encoding=UTF8&pd_rd_i=B0194IQGNS&pd_rd_r=0ecee2d4-89eb-11e9-b7d4-81d21b9098f6&pd_rd_w=EbzjV&pd_rd_wg=c9fNP&pf_rd_p=07e3e597-b71b-4701-a3fd-d79c50f48406&pf_rd_r=946PJ0RNW6FCBF4P9QXP&psc=1&refRID=946PJ0RNW6FCBF4P9QXP
I'm thinking it does not need to be too deep.Maybe something like this?
https://www.amazon.co.uk/Stewart-Blenheim-Barrel-Copper-Effect/dp/B0194IQGNS/ref=pd_bxgy_86_img_3/260-6684593-2651665?_encoding=UTF8&pd_rd_i=B0194IQGNS&pd_rd_r=0ecee2d4-89eb-11e9-b7d4-81d21b9098f6&pd_rd_w=EbzjV&pd_rd_wg=c9fNP&pf_rd_p=07e3e597-b71b-4701-a3fd-d79c50f48406&pf_rd_r=946PJ0RNW6FCBF4P9QXP&psc=1&refRID=946PJ0RNW6FCBF4P9QXP
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I already have a fish pond and wildlife ponds just wanted something for a couple of bog type plants that don't suit the other ponds.
I have ordered it anyway and if it's too small I can always use it somewhere else!!
"You don't stop gardening because you get old, you get old because you stop gardening." - The Hampshire Hog