I’ve never understood the chick hatching total in theory couldn’t I just hatch enough eggs to get a pair of f1 and then breed those ? (Forgive me for my ignorance). And also thank you that will definitely save me from a google rabbit hole in the future but I’m also curious why such a thing would happen with pure birds ?
For F1, yes you are right. You cross the two breeds, and get a bunch of chicks that look very much alike. Look for any obvious issues (like a single comb when you want rose), but you can pretty much pick a male and a few females and start breeding them together to produce F2 chicks.
The F2 generation is the one where you want to hatch large numbers of chicks, because there are so many different combinations of genes and you only want a few of them.
Of the F2 chicks, about 3/4 will be black (maybe with leakage or diluted by lavender), and only about 1/4 will be able to show any kind of pattern (lacing or otherwise.)
Of the F2 chicks, about 1/4 will show lavender, and the other 3/4 will not.
So for just those two genes, you would expect about 1 in 16 chicks to show both lavender and some kind of pattern. If you hatch 160 chicks, you might have about 10 of those birds that have both lavender and some pattern, and then you look for the best laced one among them. But you also want rose comb, and hen feathering for the males, and various other traits, so you probably need to hatch more total chicks, to have more chances of getting the "right" ones for the next generation. This is where hatching very large numbers of chicks can make your project go much faster.
Once you find an F2 chick that shows both lavender and any pattern at all (not all-black or all-lavender), you can breed it back to a Sebright to get chicks that carry lavender but have more of the other genes you want from Sebrights. Then cross two of those offspring together to get some chicks showing lavender. Then you can cross one that shows lavender back to Sebright again... and keep repeating until you've got birds with all the traits you want. This takes more generations but less total chicks, as compared with hatching F2 chicks until you find some that are really good in all traits.