Too many cookies in http header

Set-Cookie and Cookie are not the same headers.

Set-Cookie headers come from the server and tell the client to store a cookie.
With every new request to the server, the browser will send back all previously-stored cookies to the server using the Cookie header.

While there can be only one Cookie header, it may contain a large number of name=value pairs, each representing another cookie. It depends on the user agent, but RFC 6265 suggests a limit of at least 3000. But this is in reference to browsers and Cloudflare obviously feels different about their "appliance".

If you are only seeing one from the list in your post it is probably because they all have the same name. As the name serves as the index into the cookie array you're only going to have one value assigned to that index.

I believe that Cloudflare limits headers to protect against certain DOS attacks. I thought the limit was 32KB, not 8KB. But I'm no Cloudflare expert.

All that said, I'm sorry to say I don't know how to help you. It looks like all those set-cookie headers are related to sessions so you might look through the code for superfluous attempts to create sessions.