As the results stand now, Trump needs to invalidate 37 electoral college votes from Biden's win to flip the result.
Looking at the states with the closest margins:
GA: Biden up by approx 13,000 - 16 EC votes
WI: Biden up by approx 10,000 - 10 EC votes
AZ: Biden up by approx 15,000 - 11 EC votes
These are the 3 closest elections that would get Trump over 270. Assuming every fraudulent vote was in Biden's favor, Trump needs evidence of 38,000 illegal votes, at a minimum, in the exactly right places to flip the election.
That's the lowest possible bar to clear. The margins in other states are higher. PA flipping means finding 40,000 illegal Biden votes. NV means finding 36,000 illegal Biden votes. Michigan is over 100,000.
Election fraud at this magnitude would leave behind huge amounts of evidence.