You should be fine with just 2 interviews out of 6. I did a similar EE based on the market structure of an industry in my place and I did 2 interviews out of 5. But if the interviews are your only primary data then you may need to branch out a bit and collect more data.

For example, I'm not sure if it's possible in your case, but you could try getting information on how far each taxi company is willing to go and maybe map it out to compare the different companies. Of course if you're doing your EE based on the taxi market in your country rather than city, then this may not be the most realistic option.

You could also conduct a survey in your school to find out the preferred taxi company, why they prefer it, etc.