The email for batch preference went out around Sept. 19 (that’s when I go mine. Some may be earlier, some may be later).
For the actual batch you’ll be placed in, that should happen close to the actual date they ship your phone.
It’s also possible that it ended up in your spam filter (or somewhere other than your inbox). It didn’t for me, but your mileage may vary.
If the email is lost to the ether, you should be able to email ops@puri.sm with your order number and prefered batches and get squared away.