Suggest as a rough calclation 1 cubic metre converted to inches = 39.37in x 39.37in x 39.37in
1 cubic metre = approx 61000 cubic inches
your boxes are 15 x 15 x 23 = 5175 cubic inches
so 61000 / 5175 is approx 11.7 boxes
If your boxes have to go inside a crate , container or bigger box exactly 1 cubic metre you may struggle to get 12 depending on how you pack them in the box / crate
If they are free standing in a 1 cubic metret space in van then 12 would be close enough
I think my maths are right