If you want to make a token or ethereum transfer you don´t really need a lot. I would buy 5$ or 10$ of ethereum and that will cover around 20 very quick transactions or more.

The exception is if you plan to use decentralized exchanges, because you will need gas to put orders and move to wallets, etc... In that case I suggest that you get at least 20 USD of ethereum, just to be on the safe side.