How much fish to buy?

Excuse me, could you please help me with a question about fish purchasing? I'm trying to figure out how much fish to buy for a specific event or occasion, but I'm not sure how to calculate the right amount. What factors should I consider when determining the quantity of fish I need? Is there a standard rule of thumb or a specific formula I can use to ensure I have enough without overspending or wasting food? Your guidance would be greatly appreciated.