I would say choose at least 3. Do more if you really want, but it's not necessary as far as using the inventions as a learning tool. They should not be used to only develop technique, that is not the purpose.
No student can successfully champion a 3 or 4 part fugue without being introduced to those with 2 parts. And, just because it is two parts (and I'm referring to E minor from WTC 1) does not mean the fugue is ANY easier to play. Forget about tempo and focus on the clarity between the VOICES.
Best wishes,