There is no official rule as to how much shrimp a 1 year old can have but like with any other food I would not recommend to go overboard. Once you know you are clear of allergy then start by serving a handful of shrimps and see how it goes. Naturally if your child is asking for more you can give them. It's more of a question how often it should be served and with shrimps you just follow the recommendation for fish and seafood of 2 - 3 times per week.