You can get the maximum like this:
>>> import pandas as pd
>>> df = pd.DataFrame({"A": [1,2,3], "B": [-2, 8, 1]})
>>> df
A B
0 1 -2
1 2 8
2 3 1
>>> df[["A", "B"]]
A B
0 1 -2
1 2 8
2 3 1
>>> df[["A", "B"]].max(axis=1)
0 1
1 8
2 3
and so:
>>> df["C"] = df[["A", "B"]].max(axis=1)
>>> df
A B C
0 1 -2 1
1 2 8 8
2 3 1 3
If you know that “A” and “B” are the only columns, you could even get away with
>>> df["C"] = df.max(axis=1)
And you could use .apply(max, axis=1)
too, I guess.