Each tablespace has one or more datafiles that it uses to store data.
The max size of a datafile depends on the block size of the database. I believe that, by default, that leaves with you with a max of 32gb per datafile.
To find out if the actual limit is 32gb, run the following:
select value from v$parameter where name = 'db_block_size';
Compare the result you get with the first column below, and that will indicate what your max datafile size is.
I have Oracle Personal Edition 11g r2 and in a default install it had an 8,192 block size (32gb per data file).
Block Sz Max Datafile Sz (Gb) Max DB Sz (Tb)
-------- -------------------- --------------
2,048 8,192 524,264
4,096 16,384 1,048,528
8,192 32,768 2,097,056
16,384 65,536 4,194,112
32,768 131,072 8,388,224
You can run this query to find what datafiles you have, what tablespaces they are associated with, and what you've currrently set the max file size to (which cannot exceed the aforementioned 32gb):
select bytes/1024/1024 as mb_size,
maxbytes/1024/1024 as maxsize_set,
x.*
from dba_data_files x
MAXSIZE_SET is the maximum size you've set the datafile to. Also relevant is whether you've set the AUTOEXTEND option to ON (its name does what it implies).
If your datafile has a low max size or autoextend is not on you could simply run:
alter database datafile 'path_to_your_file\that_file.DBF' autoextend on maxsize unlimited;
However if its size is at/near 32gb an autoextend is on, then yes, you do need another datafile for the tablespace:
alter tablespace system add datafile 'path_to_your_datafiles_folder\name_of_df_you_want.dbf' size 10m autoextend on maxsize unlimited;