This must be homework ;-) So I'll give you some hints...
Since I haven't done tuple relational calculus since college (http://en.wikipedia.org/wiki/Relational_algebra), here is an approximation in sql,
select instructor.ID, instructor.name, count(teaches.ID)
from instructor
join teaches on teaches.ID = instructor.ID
and count(teaches.ID) >= ...
group by ...
Leaving you to fill in the group by and >= values.
Think about how you calculate how many courses each teacher teaches,
select teaches.ID, count(*)
from teaches
group by teaches.ID
This might help: MySQL count maximum number of rows