Up to 32 secondary, hash, and join indexes can be defined for one user data base table. Each multicolumn NUSI defined with an ORDER BY clause counts as two consecutive indexes in this calculation. See Why Consecutive Indexes Are Important For Value-Ordered NUSIs.
Permitting all secondary index operations for one table to be performed at the same time allows the action to be treated as a single transaction; that is, either all of the indexes are created, or none of them are created.