Remove the rows from pandas dataframe, that has sentences longer than certain word length

First split values by whitespace, get number of rows by Series.str.len and check by inverted condition >= to < with Series.lt for boolean indexing:

df = df[df['Y'].str.split().str.len().lt(4)]
#alternative with inverted mask by ~
#df = df[~df['Y'].str.split().str.len().ge(4)]
print (df)
   X               Y
1  1        An apple
2  2  glass of water

You can count the spaces:

df[df.Y.str.count('\s+').lt(3)]

   X               Y
1  1        An apple
2  2  glass of water