to repair a tear on cover?

From the help center:

Cover not significantly torn, or chewed/gnawed at all

• some used book stores cut out a small part of the cover; if the amount missing is less than 1 square inch, this is OKAY
• a small rip (less than 1.5 inches) in the cover is OKAY
• yes, that does say "chewed/gnawed" above.  That means no pet-chewed (or human-chewed) books.

So, the tear needs to be less than 1.5 inches whether repaired or not.

I hate tape. Will put it in comments even if I do not RWAP.

It does not age well. Books really need the acid free library tape for any repairs.

Tape is allowed. There is no limit.

However, the underlying tear may not be more than 1.5 inches long, or the book is unpostable, repaired or not.

If a book is unpostable, tape cannot make it postable again. If a book is postable, tape does not make it unpostable.

What Sara said

You can use all the tape you want.  But if the book is unpostable without the tape, then tape cannot return it to a postable condition (see the excerpt Melissa posted).