How do you check the returned data? Because TextFields have a 64k limit. Every string that is displayed in the Unity console has a limit of 64k most likely like the TextField components in Windows. I don't think they limit the fields-count. If there is a limit then probably a byte-limit. However, HTTP uses data-streams so there's no limit in the size you can send / receive. Most webservers / PHP configs have setup a limit to prevent overloading the webserver's memory.

I see no reason for Unity to limit the returned data. Have you checked www.bytes.length ?

If you have trouble to spot the error i recommend WireShark (a packet analyser) to see what get's actually transfered and what is returned.

I'm actually getting php to echo a single variable $dataLength which is the number of fields that I created in unity at runtime. The dataLength shows me a reading up until 999 addfields. It prints dataLength=999(I use saveData_post.text to print this single var in the console) If I create 1000 or more addfields in Unity then php just returns dataLength= (without a reading).

Unity doesn't have a field-count limitation (Just looked into the WWWForm class with ILSpy ;) ). Mayby your web hoster limit the post-var-count like mentioned in this forum. To check this you could setup your own webserver (xitami is a nice one ;) ) and test it in a controlled environment.

I would recommend to merge the data into fewer fields and parse them in php. However there's also a max-post-size limit in php which is normally set to something between 8MB and 512MB but that's completely up to your hoster.