@quid Four is the upper limit. Four other sites + the site's own meta (that is a migration path always available on every site). The minimum is zero. — Adam Lear ♦ Jun 28 '13 at 0:37

@MartinSleziak: According to the migration statistics, in the last 90 days, 39 questions have been migrated to math.se, six to stats.se, two to scicomp.se and one each to mo-meta, to dsp.se and to hsm.se. This suggests that migration to targets other than math.se and stats.se happens pretty rarely, and can well be done via flags for moderator attention. — Stefan Kohl Aug 9 '18 at 12:26