what happens if i only get invited to one pref par

You automatically get a bid from that chapter

exactly. you can get released before pref round, but if you make it to pref round AND maximize your options that night (even if that's only one house) you are guaranteed a bid.

As long as you don't break any rules (try to get a sister to bid promise or dirty rush you, are very rude, etc.), you will not be released from rush. If you maximize your options and follow the rules, you are guaranteed a bid regardless of the number of pref parties you attend. If you attend only 1 party, then you will get a bid from that house if you maximize options. If you attend 2 parties, then you are guaranteed a bid from 1 of those if you maximize your options. I hope this makes sense. If you are still confused, try asking your Gamma Chi. This might be a question that is more easily answered in person.