Depending on who is relegated and even then I can't see it being too much of a concern as it's either West Ham or Spurs that will bring the numbers. Depending if Bradford City overturn the deficit in the playoffs and win them which will be a close "rival" between them and Lincoln and neither are going to have huge numbers where they are overspilling.
If you are just thinking of home matches you will be fine without any membership however the thing to remember is on previous years
https://www.sufc.co.uk/tickets-and-memberships/loyalty-points To quote the website... "To be eligible to earn and use loyalty points, you must be a current Season Ticket Holder or Member" So if you don't have either your well earnt 30,000 doesn't count as they are frozen unless you are a current season ticket holder or member. But realistically, I can't see any games fully selling out next season unless it's either West Ham or Spurs at a decent kick off time.