One at 19th and Q in addition to the one at 4th and I. The track there is not currently used by the San Joaquins, and that station is a block north of the light-rail overpass, an overpass halfway between 16th and 23th St. stations.
Amtrak Station Planned For Midtown « CBS Sacramento has a list of possible stations. From that one and the video in the previous link, I find, from north to south:
- Natomas / Sac Airport
- North Sac
- Midtown Sac
- Sac City College
- Elk Grove
Looking northward, there is one active line, and while a North Sacramento station is possible, a Natomas or Airport one is not -- too far away. However, the airport is at a proposed extension of the Sac light-rail system.
I've also seen mention of the Altamont Commuter Express going out to this proposed station, meaning that it would be extended northward from Stockton.