It seems to me that either you have many objects in that tablespace or you have few objects with many small extents.

To alter tablesapce maxextent caluse you can use;

ALTER TABLESPACE ts_name STORAGE (MAXEXTENTS n);

Also check the INITIAL and NEXT clause of the index you are going to create. Rebuilding the existing indexes may also free-up some extents in the tablespace.

HTH

Sanjay