The minimum is whatever you want, but it's got to be delivered on a pallet via freight. It will probably cost something like $90 for the shipping. With that, the threshold to make it worth it is something like 12 sacks. Any less than that and you might as well pay $60 a sack at the LHBS. You can pay a $75 premium for a truck with a lift gate for a residential delivery.
Also, if you can be sure you'll have like 4 guys around at the time of delivery, you might want to take a chance at a hand unload where you cut the pallet open right on the truck and toss the bags down quickly. It carries a little risk though because the last bill of lading I signed off on said "driver: collect $125 if hand unload". Now, I'm not much of a gambling man but I'd say throwing the driver a $20 would make him forget all about it.