how can i compute the correlation matrix mxn?

Correlation doesn’t need to be MxN. all you are doing is checking correlation between N columns so it will be NxN matrix. From the N*N you can consider the ones which you like and neglect the others.

import seaborn as sns
import pandas as pd
from io import StringIO
df = pd.read_csv(StringIO('''TWEET, A1, A2, B1, B2, B3
tweet text, 0.23, 0.54, 120, 60, 39
tweet text, 0.33, 0.7, 70, 20, 36
tweet text, 0.8, 0.41, 68, 52, 29
'''),sep=',')
print(df.corr()) # Pandas correlation matrix
sns.heatmap(df.corr(),annot = True)

Output:

    A1              A2          B1         B2          B3
A1  1.000000    -0.732859   -0.661319   0.167649    -0.991352
A2  -0.732859   1.000000    -0.025703   -0.793614   0.637235
B1  -0.661319   -0.025703   1.000000    0.628619    0.754036
B2  0.167649    -0.793614   0.628619    1.000000    -0.036827
B3  -0.991352   0.637235    0.754036    -0.036827   1.000000