import pandas as pd

df = pd.DataFrame({

'event':[None, None, 'CRP', None, None, None, 'CRP', 'CRP', None, None, None, None]

})

print(df)

df['tile'] = (df['event'] == 'CRP').cumsum()

print(df)

Result

event tile

0 None 0

1 None 0

2 CRP 1

3 None 1

4 None 1

5 None 1

6 CRP 2

7 CRP 3

8 None 3

9 None 3

10 None 3

11

None 3