iRule is called many times for the same multipart/form-data form post

iRule is event based scripting. For example, "when HTTP_REQUEST" indicates that the iRule will be triggered whenever an HTTP Request is received by the F5 from the client. Based on the logs that you provided, you seem to be logging a lot of information like individual HTTP headers. This logging does not indicate the number of times the iRule is triggered. You can look for the number of times an iRule is executed under the iRule statistics.