Repacks are done across a set of Builders per platform. The number of Builders per platform is controlled by the 'l10nChunks' release config variable, with a default set as DEFAULT_L10N_CHUNKS in [http://hg.mozilla.org/build/buildbotcustom/file/tip/process/release.py process/release.py]. Each Builder performs one Build; each Build repacks (totalLocales / l10nChunks) locales.
==== Chunked repacks ====
If the job fails before any locale has been processed you can just hit "rebuild".
==== Standalone Repack Builders ====