To make all the datafiles of the database or a tablespace in autoextend mode

Check the current status on autoextensible

select file_name, tablespace_name, autoextensible 
from dba_data_files 
where tablespace_name='USERS';

To make all the datafiles of the database or a tablespace in autoextend mode

set feedback off
set lines 200
set pages 0  

spool autoextend.sql

select
   'alter database datafile '|| ''''||  file_name||   ''' '||   ' autoextend on maxsize unlimited;'
from
   dba_data_files
;

@autoextend.sql

set feedback on
set pages 40 

then run the  autoextend.sql

to run only for a tablespace

select
   'alter database datafile '
   || ''''||  file_name||   ''''||   ' autoextend on maxsize unlimited;'
from
   dba_data_files
where
tablespace_name ='USERS'

; 

output will be like as below :-

alter database datafile '/database/data/USERS_00_1.dbf'  autoextend on maxsize unlimited;