Kinda looks like babble fish translated this but I'll try and give it a go.

see i have a datafile which is 7.5 GB now. I have added so far 6 datafiles to exitsing tablespace with alter tablespace add datafile command.

I don't think this matters but.. if one is 7.5gigs how big are the others to the corresponding tablespace?

Now the export backup .dmp FILE SIZE of this schema( only this schema ) comes to almost 3 Gb.
If you already have one datafile out there that's 7.5 gigs and you're export size for the whole schema is only 3 gigs you're either very lucky (and thank yourself for it) or you're not exporting rows, in which case I hope this is not your primary means of backup.

I query this command from view dba_free_space
select sum(bytes)/1024/1024,tablespace_name from dba_free_space group by tablespace_name;
The tablespace FREE space now is 450mb it means that when it reaches almost 50mb i have to add another datafile ' ie 7th datafile'
If you have only 450 megs of space left on that tablespace and you're in a production env, you should of made the tablespace bigger a long time ago. The thing I would ask is, how fast is this tablespace growing? Is there data being loaded on there that is not necessary and can be either deleted or archived off.
But to answer your question without knowing the whole story I would add another datafile to the tablespace and make it big enough where you're not going to have to worry about space for awhile



now my question is will tablespace AUTOEXTEND option help in this scenario.
or
manually adding the datafile will help me - because u see it helps me to monitor the space growth
Using AUTOEXTEND in my opinon is a matter of the DBA's choice. Personally I hate it. I much rather keep tabs on the tablespace myself to watch growth rate and to keep tabs on the developers when they decide to make some load.