In R: I want to check how many times one value of df1$A is in df2$A

Question

I have two dataframes.

df1 
A B
1 x
2 x
3 x
4 x
5 x
6 x
7 x

And df2
A 
1
1
1
2
2
1
5

Not I want to check how many times one value of df1$A i