Line Plots · Hard problem

Two classes each made a line plot of the number of pets their students own. Class A: above $1$ there are $2$ X's, above $2$ there are $4$ X's, above $3$ there are $3$ X's. Class B: above $2$ there is $1$ X, above $3$ there are $5$ X's, above $4$ there are $2$ X's. The classes combine their data into one line plot. How many X's are in the tallest stack of the combined plot?