1
11
21
1211
111221
312211
13112221

What number comes in the next row?

Every line describes the line before it.

For example the second line that describes the first line says: One 1
Therefore it will be written down as 11.

Now the third line which describes the second line has to be
Two ones. So it will be written down as 21.

The last row in the puzzle is 13112221. This is 1 one, 1 three, 2 ones, 3 two's, 1 one.
So the next row will be: 113213211