I have no idea, I hope someone else has tried this.

I hate to admit I've never even eaten an artichoke, let alone grown one. A quick Google search gives ranges from 18" apart up to six feet apart! So the very least you could get by with would be 18", or maybe 9 per bed. It might be safer to put in only four. The link below gives the 18" spacing.

Granny

Here is a link that might be useful: Growing Artichokes