All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad, is His slave and Messenger.
There is no specific amount for what you asked about, but in such cases, one should buy his needs without harming others or negatively affecting market prices or any other public interests. An-Nawawi said in his book Sharh Saheeh Muslim: “However, if he wishes to buy some goods from the marketplace and store them as sustenance for his dependents at times of food shortage, it is impermissible. Rather, he should buy food without causing hardship for other Muslims; he may buy food which would be enough for some days or a month. If there is no food shortage, he may buy food which would be enough for a year or more. This opinion was transmitted by Al-Qaadhi with such detailing on the authority of most scholars.” [End of quote]
Allah Knows best.