Count in relational algebra using Ω = { π, σ, ⋈, ⋉, β, x, ∪, ∩ , - }
Hint: to express "the set $S$ has size $\ge 1$" in propositional logic (without using the "size" operator), you can write $\exists x . x \in S$.  How would you express "the set $S$ has size $\ge 2$"?  Does that give you any ideas for how to use relational algebra to solve your question?