Welcome to KH and to knitting!
This pattern is written for different sizes. You would increase at each end of the 5th row and then at each end of the following alternate rows or the following 4th rows depending on whether you’re making the 1st, 2nd, 3rd, etc size.
The following alt rows would be 7,9,11,13 and so on.
The following 4th rows would be 9,13,17,21 and so on.
Somewhere at the beginning of the pattern is a guide to the different sizes, either the baby’s age or measurements for each of the sizes. Once you know which size you want to make, you can circle or mark that size throughout and follow those instructions.