One table with many rows or many tables with a few rows I'm having a question rounding my mind. It's better to create one table and store a lot of rows there or create various tables and have a few rows on them? What's the performance and the I/O saved at physical level? How many space will use many tables instead of one table?  I'm thinking in how Wordpress handling the multiblog feature because I'm starting a new project and I'm looking for the best approach.
Assuming that you are talking about tables containing the same kinds of entities, you typically want to have one table.

You would not have any performance differences and a whole lot of management differences between the two approaches, with the single table being easier to manage.  Typically large tables do not have performance overheads compared to smaller versions of the same when properly indexed, since the index seek time grows slowly compared to the row growth.

In a normalized design, you will have different tables for different entities or relations, but as far as partitioning (either supported with a DBMS feature or manually with separate tables), that usually is necessary when you have certain requirements, like granularity of backup or data loading/unloading on a partition basis.