You might be better switching things around and telling us how big the area is that you want to cover with mulch.
You need to multiply length by width by depth. So for example, if your garden bed is 4m long by 3m wide and you are mulching to a depth of .1m, you would need 1.2 cubic metres of mulch. It's a simple calculation of length x width x depth.
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Typically, it would be best to lay mulch at least 40mm deep to achieve decent coverage. Any thinner, and it will look sparse, allowing you to see the ground through it. If you lay down a 40mm thick layer, your 1m3 of mulch would cover 25 square meters.
I hope this helps. Please let us know if you have further questions.