Answer to ur Question

First datafile was created with 2gb and when the tablespace was getting full i added 6 additional datafiles with 1 GB each everytime whenever i had to .

everytime when 1 GB space was added i didnt worry for a while.
Till 2 months it is ok. Then next 3rd month i have to add additional 1 GB space.

The database is in archive log mode and Hot backup is the primary mode.

Export backup is 3 Gb size and it includes all rows & data for that schema.

i personally like to add datafiles when the tablespace gets full, because when i add i know and i can track it by querying dba_free_space.
so now i feel is whenever i have to add a datafile instead of 1 GB i have to give 3 or 4gb and see it.