IIRC, Dec. 14 is the date that each states electors must be appointed and cast their votes, and Jan. 6 is when the new congress counts and certifies the electoral votes.
It doesnt matter what Trump says, it matters what he does...
Im sure he will have lawyers challenge the results in any states with close margins, but most states dont allow recounts unless the margin falls below 0.5%. Thats why Biden must win the swing states with a margin that can withstand any challenges. Trump will try to challenge mail in ballots as fraudulent, but I dont think that will get far in court.
There has been some concern about states with Republican legislatures refusing to appoint electors, but in many states, the legislature has no power over the appointment of electors, it is the role of the Secretary of State to certify the popular vote and appoint electors as prescribed by law. In Michigan, for example, the SoS must appoint electors based on the outcome of the popular vote.
If Trump or any states GOP legislature succeeds in blocking the appointment of any electors, that just lowers the total needed to win the presidency. Some have worried that this strategy would throw the decision to the house, where each state gets 1 vote, but that only applies if neither candidate gets a majority of the appointed electors, which is not possible if electors are split between just 2 candidates- except for a tie (the times in the past that the house has had to decide elections has only occurred when 3-4 candidates earned electoral votes or there was a tie).
If things turn out as projected in current polls, Trump will have to brazenly attempt to overturn the will of the people in numerous states- a strategy not likely to succeed, IMO.
Once the EVs are certified by congress, Trump can wreak havoc as a lame duck, but he is powerless to avoid the reality of his term ending on January 20, 2021.