Can I say no when my principal investigator asks me to help cover lectures next semester?
I don't think a flat "no" would go over very well.  Covering classes is something that colleagues are expected to do for one another, within reason.  Yes, everyone has their own projects that this takes time away from.  But it's really the only way that faculty are able to travel during the school term, which most find essential to keeping an active research program.  Saying "I don't want to do any lecturing" will come across as unreasonable and possibly selfish.  Keep in mind that "focusing solely on research" is a luxury that basically nobody ever has.

However, you can certainly negotiate, and maybe reduce the amount, while still showing that you're willing to help.  Keep in mind that the PI presumably has an interest in your research success, so if the teaching would impact this in specific negative ways, beyond a generic "this is an hour of my time that I could be spending on research", you can point this out and suggest a compromise.


"I'm happy to help out, but seven classes over the semester is kind of a lot.  I am going to need to [achieve specific research goals] next semester and I think this might start to get in the way.  Maybe you can find another person who can cover some of them, and we'll split them up?"
"I can do January 18, but on March 7 there is a conference that I was really hoping to attend.  Perhaps someone else could cover that date?"
"February 10 would be fine.  April 3 is the week before [important deadline] and it might be kind of a crunch, so maybe someone else could teach on that day."


You can also ask for materials or other help that will lessen the amount of work for you.


"Sure, I can teach on January 30.  Can you point out the specific sections in the book that I should cover?  Will you be preparing notes for me to follow, or do you have notes from a previous semester?  Can you suggest any particular examples that I should present?  [etc...]"


With such assistance, covering a lecture should not end up taking an excessive amount of time away from your research.  You really should only need to prepare and deliver the lecture - you likely won't have to deal with creating assignments, grading exams, supervising TAs, holding office hours, answering emails from students, or the other things that end up taking the majority of time when teaching.