Numpy Size Count

import numpy as np
import pandas as pd
df = pd.DataFrame({'a':[0,0,1,2,2,2], 'b':[1,2,3,4,np.NaN,4], 'c':np.random.randn(6)})
df
a b c
0 0 1.0 -0.600733
1 0 2.0 -0.399252
2 1 3.0 1.713596
3 2 4.0 1.211463
4 2 NaN -0.120609
5 2 4.0 -1.763791 
df.size
18

df.count()
a    6
b    5
c    6
dtype: int64

df.groupby('a').size()
a
0    2
1    1
2    3
dtype: int64

df.groupby('a').count()
b c
a
0 2 2
1 1 1
2 2 3 
df = pd.DataFrame({'A': ['x', 'y', np.nan, 'z']})
df
A
0 x
1 y
2 NaN
3 z 
df.A.size
4

df.A.count()
3

More:

https://stackoverflow.com/questions/33346591/what-is-the-difference-between-size-and-count-in-pandas