Generally patterns are printed in several different sizes. The numbers in ( ) are for the different sizes.
For example, in the case of 19 (21,23) if the pattern is Small Medium and Large the first number 19 is for the small the 21 for Medium and the 23 got large. You select your size and use the number that corresponds.
GinnyG on Ravelry, I'd LOVE to be in your "friends"
Usually size small (medium, large, me.) In other words, in your example, if you're knitting a small whatsis, knit 19, purl 1, knit 39 and purl 1. If you're knitting a large, knit 23, purl 1, knit 46, purl 1. It helps to go though your pattern and highlight the appropriate number of stitches for your size.
Sometimes you'll see:
Row 3 K1, yo, k2 tog, k to end.
Size Small: Row 4: P2...(and a bunch more directions)
Size Medium-Large-XL: Repeat Row 3 1 (2,3) time(s)
In that case, with a size small, you go on to the next part of the pattern; with medium, large and XL, you add repeated rows to add width or length. Once again, it saves a lot of frustration if you go through your pattern first and highlight the directions for your size.