How many mm of nasal nostril could be reduced from my nose? (Photo)

Thank you so much for submitting your question. 
The ideal alar width is approximately the same as intercanthal (width of your eye) width. Looking at your photos I would agree you do have widening of your alar region. TO narrow it will likely require external incisions along the exterior of the nose called alar base resection. The degree to which you can narrow this is based on being able to maintain the external valve of the nose for breathing purposes. A surgeon in person could review this more with you. 
Best of luck and I hope this answers your questions. 

Based upon your photo, you could probably be able to achieve a result where nostril width would be reduced to approximately half the size of your present nostril width through Rhinoplasty with Nostril Narrowing.  I would suggest a formal in person evaluation with a facial plastic surgeon who is an ethnic rhinoplasty specialist with extensive experience in nostril narrowing surgery.  Make sure you see numerous before and after photos with results you like and seek.  Request computer imaging.  This step ensures that you and your surgeon are united as to the desired end result as well as making sure the end result can realistically be achieved.Best wishes,Dr. S