Matthew McGiffen dives into rules of thumb:
I find this is a question that comes up again and agan. What estimate for the number of rows returned does SQL Server use if you’re selecting from a column where there are no statistics available?
There are a few different algorithms used depending on how you’re querying the table. In this post we’ll look at where we have a predicate looking for a fixed value.
Read on for a few examples, noting that this specifically relates to tables and not things like table-valued parameters.