Formal:
* Minimum of - "The minimum of five people must be present for the meeting."
* No fewer than - "There were no fewer than ten cars in the parking lot."
* Not less than - "The project will take not less than six months to complete."
* As many as - "There were as many as 200 people at the concert." (This implies a possibility of more, but definitely at least that number)
Informal:
* At the very least - "At the very least, we need to finish this report by Friday."
* At a minimum - "At a minimum, you should be able to write a basic email."
* At the lowest - "The cost of this project will be at the lowest, $10,000."
* At the very least - "At the very least, we need to start thinking about our next move."
* At the very minimum - "You need at the very minimum, a good night's rest."
To emphasize a low amount:
* Barely - "We barely managed to get the project done on time."
* Just - "I just got enough sleep last night."
To express a minimum requirement:
* Must - "You must be at least 18 years old to vote."
* Need to - "We need to have at least 10 people sign up for the event."
The best option depends on the specific situation and the desired level of formality.