Pandas Excel groupby/count

Keys

1. Convert the comma-separated string into lists first.
2. Use df.explode() to expand the entries.
3. Pivoted aggregation (to which concept that group size belongs) can be achieved by df.pivot_table().

Setup

df = pd.read_csv(io.StringIO("""
Labels                        Labs
a1, b3                         1
a2                             3
b3                             1
"""), sep=r"s{2,}", engine="python")

# split string into list (assume consistent separator pattern)    
df["Labels"] = df["Labels"].str.split(", ")

First output:

df.explode("Labels").groupby("Labels").size()

Out[69]:
Labels
a1    1
a2    1
b3    2
dtype: int64

Second output:

df.explode("Labels").pivot_table(index="Labels", columns="Labs", aggfunc="size")
    .fillna(0).astype(int)

Out[70]: 
Labs    1  3
Labels      
a1      1  0
a2      0  1
b3      2  0