The short answer is this: For every dwelling unit within the Alki Area Parking Overlay (see map) developers are required to provide 1.5 parking spaces.
For the long answer, follow the link below to Chapter 23.54 of the Seattle Municipal Code. When you get there, you will find a long document that explains parking requirements for buildings and institutions throughout the city. For specific mention of the Alki neighborhood, scroll down to Table B, "Parking for Residential Uses," then scroll down to item "O."
Subtitle III - Land Use Regulations
Division 2 - Authorized Uses and Development Standards
Chapter 23.54 - QUANTITY AND DESIGN STANDARDS FOR ACCESS, OFF-STREET PARKING, AND SOLID WASTE STORAGE