As noted in the pre-order information about NBA 2K18, there is a line item for "weekly MyTEAM packs" in the game's Legend Edition and Legend Edition Gold. We've received a few inquiries asking for more details about these packs.
A weekly MyTEAM pack is a complimentary MyTEAM pack that you'll receive, once a week, when you log in to the MyTEAM mode. You'll only receive one pack at a time. It should be noted that these packs will not accrue over time. That means if you don't log in one week and then do so the following week, you're only going to receive one MyTEAM pack.
That said, missing a log in and a pack one week will not count against the number of packs you still have available. You'll continue receiving your free packs on a weekly basis until you've redeemed the total your pre-order qualifies you for.
TL;DR: You get one free MyTEAM pack per week when you log into MyTEAM. Skipping a week won't net you two packs when you log in next; you'll only get one. You'll keep getting a free weekly MyTEAM pack until you've received all the ones your pre-order entitles you to.