The question would be better if it asked “where can one find the data on…”. As discussed in the comments, as written the answer to your question is either “no one knows because it did not happen yet” or “here are the numbers and when someone looks tomorrow they will be inaccurate.”
That said, the data you want with requested, sent, returned by party affiliation and county can be downloaded as an .xlsx on https://www.vote.pa.gov/About-Elections/Pages/Election-Results.aspx
Scroll down and click on “Daily Mail Ballot Report”. Despite the name, it has tabs for total, mail in, and absentee.
As the PA Board of Elections themselves describe it:
This point-in-time transactional data is being published for informational purposes to provide a high-level overview of the processing of absentee and mail-in ballots by county election offices. This data is pulled once per day from the Statewide Uniform Registry of Electors (SURE system) and it reflects activity recorded by the counties in the SURE system as of the time of the pull.