You can create a number of environment variables, and the total size of the set can be no more than 4 KB. The name must start with any letters from [a-zA-Z], and can contain alphanumeric characters and underscores, such as [a-zA-Z0-9_].
In addition to that, AWS Lambda reserves some specific sets of keys. You will receive an error message if you try to set values for any of the reserved keys.