Aggregation and grouping operations can't be constructed from the basic relational algebra operations. You would have to define your own COUNT
and GROUP BY
operators to perform this.
There are many proposed extensions to the basic relational algebra that you could use, or you could even define your own - but to do this formally, I suspect the mathematics would get reasonably complex.
A simple proposal (without much formality) can be found here, http://myweb.lmu.edu/dondi/share/db/relational3.pdf. (Section 3.2)
Using the extended algebra proposed in that link, your expression would be written like: