Is there a minimum number of cases the reference level has to have to be valid in the regression analysis? I have a population of about 11,000 cases. For substantive reasons (it makes the most sense to use this reference level substantively) my reference level is one that turns out to have only about a hundred cases. I prefer for analytical reasons to leave it the reference level, but I get nervous having so few cases as a reference level. There are about 19 variables in the model.

I have not seen anything for linear regression that says how many cases you need or if the ratio of cases in the reference level to one of the related dummies matters.