I guess as many as your DB (Oracle, SQL Server or DB2) can manage.
But I think that as soon as you hit the maximum column limit, you really need to to rethink your data model.
If you mean rows, I think that you have to think about archiving strategies.
If you are using an Oracle database then you can have approximately 1000 columns in one table. (ora-01792.ora-code.com)
However, if you are making such huge number of columns then you sure need to do a validation as to the need for these as this will have a very adverse impact on the performance of your siebel application as well.
Hope this helps,
Edited by: tanmay.jain on Oct 5, 2010 2:47 PM
We have seen some very large Siebel tablels.
One of our customers has nearly 1 billion records in s_evt_act.
Another customers has 1.5 billion in usage table.
Tuning Siebel with tables this large is lot of fun for us.