I currently have 12 open, yikes. And that's on my upstairs computer. Probably another 6 on my basement computer.
Upstairs I'm working on my One-Place Study, and I have FamilySearch, Find-a-Grave, BLM land patents, and WT, including WT+. One WT page is permanently open to a profile with a sticker text highlighted so I can just copy it into new profiles I create.
Downstairs I'm transcribing some African-American records from National Archives, so two pages there, FamilySearch and WT. Find-a-grave isn't very useful on African-Americans of that time period.
I don't work on both in the same day. Typically I spend a few weeks on one project, then move over to the other project, so I keep all the tabs open so I don't forget where I was at.