Build a Block matrix

Hi I have an nxn matrix called C_mx which is initialized by ones(n). I want to build a block matrix C_holder to hold twenty of these nxn matrices, five in each row (4 rows). I have twenty pair of variables that contribute to calculating C_mx. I run a for loop to calculate C_mx or each pair and I would like to store the calculated C_mx in C_holder every time. Say for the first pair, C_mx is calculated and placed in C_holder as the first block matrix. What I would also like to be able do, is that after generating C_holder for all C_mx's, I want to be able to extract let say, the 4th C_mx from C_holder. In another world, is there any way that I can index this C_holder like a 4x5 matrix, where each index refers to one of the C_mx's, so that I can point out to that specific index to get the C_mx that I want and and do my calculation? I hope I was able to clearly state what I want to do.

Thank you

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