I sometimes stream on the livestream platform Twitch. I have now set up a donation link via Streamelements. The money goes to a PayPal account which is not a business account. I think that there's no more than 50 euro a month. Do I theoretically have to register a business or pay taxes?
From 400 euro / month you have to tax income
If you made a profit in the long term, yes.
Then this income tax is and a trade must be deregistered.
Whether or how much tax you have to pay depends, as mentioned above, on the one hand on the profit (expenses from this activity are deducted from your income) but also on other income. What is relevant is how high your total taxable income is in the year. What else do you do for a living?