SingleRepetition

Use AtLeast(int), AtMost(int) and Exactly(int) to specify the bounds. If used together, AtLeast should be before AtMost. Exactly can obviously only be used alone.

var result = se
    .Letters.AtMost(3) //Zero or more, up to 3
    .Numbers.AtLeast(1) //One or more
    .Generate();

var result = se
    .Letters.Exactly(3)
    .Generate();