If I create multiple sequences within a project, and 'reset current workspace', only the sequence tab currently selected will remain (at least, this is the case when I save and reopen the project). I tend to sync video and audio on the timeline in one sequence, and build an edited scene in another, so I lose either the synced rushes or the scene-in-progress.
Does anyone knows why this would happen?
To address the OP's original question, each workspace "remembers" how many Timelines were open when it was saved. So if you have a different number of timelines open when you reset the workspace, the UI will revert to the number specified (unless, of course, the current project has fewer sequences than the number of Timelines specified).
You can save custom presets with a given number of Timelines, but that approach has a gotcha: Premiere doesn't know which sequences to load for you. Say a project with 20 sequences is open and you switch to your "Six Timelines" custom workspace. PPRO will load 6 timelines, but the odds are slim that they'll be the ones you want.
So opening the sequences as Jim and Steven instruct will probably be the easiest way to get the desired ones loaded. Note that you can open multiple selected sequences at once by right-clicking and selecting Open in Timeline.