250g of canned tuna is needed to make tuna loaf for 5 servings, how much g of tuna is needed for 8?

250g tuna are enough for 5 servings. Therefore, for each serving you need
250g / 5 servings = 50g / 1 serving.

For 8 servings you will need ( 50 * 8 ) = 400g of canned tuna.
Bon appétit!