There is no minimum credits purchase, you can buy as few credits as you want. On a side note, please consider that when you buy credits, you'll pay a transaction fee on the chain where the transaction will be executed.
To increase efficiency and save transaction fees, we recommend buying credits once a month and aiming to cover estimated fees. For example, if on average you have paid 30 USD of fees over the last months, we recommend buying each month 30 USD of Mizar credits.