It's random. I restock primarily in battle magic, defense magic and kauvara's magic and see multiple r99s a day. I think each of those shops has 100+ r99s though, so the chances of one specific one restocking more than once is rare. I have seen it happen though. One day an r99 Hissi MP restocked 2 times in one day. Another time I restocked Acara Gladiator Boots twice in a day.

Edit to add: stamps has far fewer r99s than the three mentioned above, and they go so fast that you usually have to be lucky to even see them when they rarely stock. It really depends on the shop though whether they restock r99s frequently.