How much does it impact the admission chances by getting a 34 as opposed to a 35 on the ACT?

If you are applying for incredibly competitive universities like Caltech, MIT, Stanford, and Harvard, I'd suggest that you attempt the ACT again and aim to score 35 or 36. Since, most students applying to these colleges score 34, your chances of being admitted are higher if you get a 35 or 36, considering other aspects of your application are also excellent. For less competitive colleges/universities, 34 would suffice.