I'm thinking of making a hook up pact whilst having a girlfriend
Why would you want to hook up with other people if you already have a girlfriend?
I need to spend time with a lot of people.
If you want time with other people, then you are being disloyal to your girlfriend for thinking about hooking up with other people. I think if your girlfriend finds out, you will destroy your relationship and you will come to regret it.
It's not like she doesn't know about it.
I do hope you're telling the truth and she knows about it and 100% consents that you are hooking up with other people. I still recommend that you talk about it often and the first sign that she wants you to stop, then you should also stop.