When you save a Workspace, it also saves the number of timeline panels that are displayed. All of the default workspaces that come with Premiere are designed to display one timeline panel.
If you don't want your Workspace to change the number of displayed timelines, the first thing you should do is open the maximum number of sequences you might need and then choose Window > Workspaces > Save Changes to this Workspace. (Just a tip: don’t overdo it—opening 100 sequences and saving your Workspace will slow down switching workspaces.)
If you create a workspace with 20 timelines and have 20 timelines open, it won’t close any timelines. If you have less than 20 timelines open, it won’t add any extra timelines.
However, workspaces in Premiere are always being updated, even if they're not being saved. So, if you created a workspace with 20 timelines but then closed one, the next time you switch to that workspace, it will only display 19 timelines.
The good news is that switching workspaces doesn't technically "close" that timeline - it just doesn't display it. If you go to Window > Workspaces > Reset to Saved Layout (shortcut Option/Alt + Shift + 0), it will ensure there are enough timeline panels to display all your open sequences.
TLDR:
- Switch to a workspace
- Open 10-30 sequences in the Timeline
- Go to Window > Workspaces > Save Changes to this Workspace
- If you close a timeline, this will reduce the number of timelines the Workspace will display. However, Window > Workspaces > Reset to Saved Layout (shortcut Option/Alt + Shift + 0) will restore all open sequences.