L=[[0,0,0,0],[0,0,0,0],[0,0,0,0],[0,0,0,0]]

L[0][0] = 5

L=[[5,0,0,0],[5,0,0,0],[5,0,0,0],[5,0,0,0]]

STOLBCI = STROKI = 20

def chernmatrix():
    return [[0 for _ in range(STOLBCI)]
            for _ in range(STROKI)]

import copy

# ...

stroki = 20
while stroki > 0:
    K.append(copy.copy(L))
    stroki -= 1