Based on my previous experiences, you will be fine for most of the "regular" marathon merch (pins, Tervis, magnets). The outages I've seen with that type of merch has been with the 5K stuff and the challenges...and any new types of merch (i.e. the runDisney MagicBands the first year or the 4-park Minnie Ears this year).
The only exception to the above is there is sometimes a shortage of sized items where there's not enough of a certain size that year. I seem to be recall shortages especially with the "I Did It" shirts on more than one occasion.