Hi, I learned that one must apply 7 straps for the arm tefilin but I see many people who put 8. Is this because they consider the last strap not a complete one so they add an extra one? Which practice is more acceptable?

Shalom, Thank you for your question. You are quite observant to notice such details as other people's tefillin straps - I hope this isn't an indication of lack of concentration in your own prayers! The Shulchan Aruch (Orech Haim 27,8) writes "Everyone is accustomed to wrap [the tefillin straps] six or seven windings [on the arm]". The Mishna Brurah writes that our custom is to wrap the straps 7 times. But everyone agrees that these wrappings are only custom, and do not interfere with the fulfillment of the mitzvah. In fact even if one were to leave them out altogether and go straight to the wrapping on the hand, it would be kosher. It could be that what looks like eight wrappings could very well be seven from the angle of the person wearing them - as opposed to you seeing them from the back or side. Or it may be as you write that the half wind at the end is not counted. This depends on the different ways to wrap the tefillin, whether in the Ashkanazi fashion towards the body, or the Sephardi and Chassidic custom to wrap them outwards. The correct practice is to continue the way your father puts on tefillin. If you can't find out what he did (or does - or perhaps he doesn't ... yet) continue with what you were taught. Blessings.