The key to negotiating with suppliers is to determine the market price of your desired product. Here
Unless you know your product extremely well and the cost to manufacture it (VERY few people do) your absolute only way to know the fair price of your product is to receive competing offers which is why you need to contact several suppliers.
You should at this point start to receive prices from your suppliers. If they ask you how much you will be importing, let them know your ideal annual order amount (be an optimist but don’t promise the moon) rather than the individual order amount.