What is the minimum ETH to buy?

Excuse me, could you please clarify for me what the minimum amount of Ethereum (ETH) is that one needs to purchase? I'm new to the world of cryptocurrency and I'm trying to understand the basics. Is there a set minimum limit set by exchanges or is it more flexible depending on the platform? I'm interested in investing but want to ensure I have a clear understanding of the requirements before proceeding. Thank you for your help.