The numbers of tables shouldn't impact except to the degree that each table will have a minimum size (extents), which is unlikely to be full, so you'll have more space allocated than you could otherwise use. This could impact disk access times.
However from a design point of view, it would be a mistake to have multiple tables of the same structure wither either slightly different names or in different schemas.
Every time an SQL statement goes to Oracle goes to Oracle, it needs to do a parse. If it 'remembers' running the statement earlier, it's pretty quick, otherwise its signficantly slower (ie has to work out a query plan, look at table/index definitions, statistics etc). [When I say slower, I'm talking 10ths/100ths of seconds]
If you are running lots of slightly different SQLs (eg SELECT...FROM table_fred1, SELECT....FROM table_bill2 etc), you'll be doing lots more of these slower 'hard parses', which will add up, sort of like having a heavy load in the boot of the car making it a bit sluggish.
Oracle's solution to this is called either FGAC (Fine Grained Access Control) or VPD (Virtual private Databases). Basicaly, it means that behind the scenes, Oracle can rewrite statements to add in a clause to limit what users see/affect in a table. That means that Bill can do a DELETE FROM TABLE_TRANS; and only Bill's rows will be deleted and Fred's rows in the same table will remain untouched (and invisible to Bill).
All the users share the same table and the same queries, so Oracle hs a better chance of 'remembering' the queries.