I don't think you'll be able to get the number of transactions per table. In fact, even if you could I don't think this number would actually say a lot. A single transaction can modify milions of rows in a table or it can modify only a single row. A single transaction can modify dozens of tables or it can modify only one table. And so on and so on....
What you can find out (at least approximately) is I/O activity on specific tables, which qould be much more meaningfull information. To do this, you can use script catio.sql which you can find in your $ORACLE_HOME/rdbms/admin directory. This scripts does what gpsingh sugested - it gets the I/O activty by sampling buffer cache (x$bh).
ASCII a stupid question, get a stupid ANSI
24 hours in a day .... 24 beer in a case .... coincidence?