Jo Jorgensen, the Libertarian Party candidate for US president in 2020, has said that if elected, she would pardon anyone convicted of a victimless crime.
Ignoring the fact that Congress would impeach her, that she might be lying, and that her chances of getting elected are less than 0.1%,
She does not assume a power to pardon for state crimes. The main impediment is the practical one, since a vague order regarding victimless crimes may not be enforceable. POTUS has historically granted general amnesties, such as Lincoln's Proclamation of Amnesty and Reconstruction which did not specify particular laws that were violated or individuals who violated the laws. Some degree of resistance to such an order (by prison officials and the judiciary) is to be expected, to the extent that the scope of an amnesty is ambiguous. The Lincoln proclamation is clear enough in its scope (it was triggered by taking an oath of allegiance and the persons to whom this remedy was not available are clearly-enough described).
There are on the order of 70,000 federal criminal convictions with sentencing every year, so probably over a million cases would be subject to review. She would need to sharpen the criteria, presumably by enumerating the applicable statutes. This may include or exclude convictions for perjury or lying to a federal officer. In an individual case, it could be argued that in this case, lying did not violate the rights of another person, but in that case it did – case by case review could be required. Violation of 18 USC 228 (Failure to pay legal child support obligations) might be considered to have a victim, or not, so just saying "non-violent victimless crimes" leaves open the question whether interstate refusal to pay child support is in the pardoned set. Targeted amnesties such as violation of the Controlled Substances Act could be specific enough that they could be enforced.
Legally, the biggest hurdle is that the President can only pardon those convicted of federal crimes, in their position as the Federal Executive.
Depending what is meant by "victimless crime", I would imagine many crimes are actually prosecuted and convicted under state law (e.g. drug possession, prostitution, etc.), and so are out of reach for a Presidential pardon.
Here's the relevant part of the Constitution:
"[The president] shall have Power to grant Reprieves and Pardons for Offences against the United States, except in Cases of Impeachment."
So from a Constitutional point of view, there's nothing stopping the president from pardoning a ton of people at once. There are only two restrictions: the president can't pardon state offenses and the president can't override an impeachment. That second category is very small but state offenses comprise a lot of non-violent crimes.
user6726 is correct in saying that the real challenge is practical. Issuing one sweeping pardon would be really really hard. Defining a "victimless crime" would require a large team of attorneys to choose which laws are truly "victimless", and each case would have to be reviewed to determine if the situation really fit the definition. In the real world, the Justice Department Office of the Pardon Attorney reviews applications and offers opinions to the White House. If Jorgensen wanted to pardon that many people, she could create specific guidelines and then have the Pardon Attorney review the resulting hundreds of thousands of applications (the office would have to hire a lot more people). Then she could issue pardons based on their reviews.
