Each PA school will have different criteria. You need to get used to doing the research to find out what every program that you apply to is willing to accept as hce for their applicants. It’s not one size fits all. For instance, my nearby PA program doesn’t care at all about HCE... but they like to see certain humanitarian service from their applicants. Another program nearby will not accept any HcE that isn’t of a professional licensed level... ie career level stuff like RN, respiratory therapy, medical laboratory scientists, paramedics, radiological technologists. So no CNAs, EMT basics, dental assistants. They won’t even look at your application without you having both that kind of healthcare experience and several thousand hours of it. So do some research so you aren’t wasting time and money on a program that isn’t even going to entertain your application. They all are different and there is no way to sidestep the requirement to hunt down the programs that are looking for folks like you. Nobody here can really answer that question the way that you asked it and have it be something that is helpful to you.
The answer to whether your dental asssisting counts as HcE for PA school is “it depends on the school”.