In addition to TensorBoard scanning subdirectories (so you can pass a directory containing the directories with your runs), you can also pass multiple directories to TensorBoard explicitly and give custom names (example taken from the --help output):
In recent versions of TensorBoard, aliasing this way requires a different argument, however its use is discouraged (quote from current documentation on github - linked above):
Logdir & Logdir_spec (Legacy Mode)
You may also pass a comma separated list of log directories, and
TensorBoard will watch each directory. You can also assign names to
individual log directories by putting a colon between the name and the
path, as in
This flag (--logdir_spec) is discouraged and can usually be avoided.
TensorBoard walks log directories recursively; for finer-grained
control, prefer using a symlink tree. Some features may not work when
using --logdir_spec instead of --logdir.