Setting aside the added confusion about sausages and strawberries (!), I can understand why you are perplexed by this, but it is logical if you think about it. It's all to do with the answer you are expecting. If a stallholder in a market is selling melons or pineapples, it would be normal to say 'How many do you want?' , because the expected answer is a quantity - one melon or two pineapples, for example. But if it's something grammatically countable (like strawberries) but so small that you wouldn't count them singly, you wouldn't say 'How many do you want?' , because you would hardly expect the customer to say 'I'd like 147 strawberries, please' ! I agree that 'How much..?' does sound a little odd when you start to analyse it grammatically, but it's still better than 'How many?' 'How much..?' leads on more naturally to the expected answer 'Half a kilo'. Alternatively, you could say 'How many kilos..?' or 'How many grammes would you like?'.

Because of not quantity, but weight measure.