The choice between ‘small number’ and ‘low number’ depends on the context and what you are describing.
Use ‘small number’ when you are referring to a quantity that is not large in magnitude. It emphasizes the size or quantity of something relative to its expected or possible range. Examples:
Even a small number of undecided voters could decide the winner of the presidential election.
Only a small number of applicants are successful.
The company received a small number of complaints about the new product.
There were only a small number of cookies left on the plate.
There were only a small number of people at the event.
Use ‘low number’ when you are specifically talking about a count or quantity that is low in comparison to a standard or expectation. Examples:
Australia approves record low number of student visas.
The company had a low number of sales this quarter.
The relatively low number of dead has been attributed to advance warnings.
The restaurant had a low number of reservations on a weekday.
The town experienced a low number of car thefts last year.
Photo: The National Mall, Washington, D.C.