Short answer: I’ve seen recommendations for 1oz/5gal.
Long answer: it can really depend on the quality and how much you are removing the less flavorful parts. I’ve bought lemongrass for cooking at grocery stores and specialty grocers with wildly different potency. The 1oz/5gal recommendation is a good starting point but you can always start on the lower side and add more. If you overdo it, you can’t take it back.