Take Linux: they've got a "super-long-term-support" branch, a "long-term-support" branch, a "stable" branch, and the next "stable" branch. Stable is supported until the release of the next "stable" & 3 months after that, with (usually) 2-6 months between stable releases. "long-term-support" branches are supported for about 5 years, "super-long-term-support" for a few years after that. So there can be up to 4 released branches actively supported at any given time, ignoring any feature branches.