How much ETH do I need for gas?

I'm curious, can you clarify for me what the exact amount of ETH I should have on hand to cover the gas fees for a transaction on the Ethereum network? I understand that gas prices can fluctuate, but is there a general rule of thumb or a recommended minimum balance to ensure my transactions go through smoothly? And, how does the complexity of the transaction or the current network congestion affect the amount of ETH required for gas? Thank you for your help!