another problem on this: my slave sites are showing an icon to indicate the number of tables in the database is different. opening them and saving again is "correcting" this, but after production of the slave sites i uninstall multisites on them because the customer does not have to see multisites component on is site. BUT... i see after doing this "correction", the multisites component is installed again in the slave sites... how to solve all this?

In general, the components (and sometimes, modules and plugins) are composed of 2 parts:
- My SQL tables where is stored the component content
- The PHP code that perform the tasks proposed by the extension.

The installation or the update and upgrade of an extension in the master result by the installation of the PHP code and also the MySQL tables.
In the case of an upgrade or migration, the extension may change the MySQL tables layout and content.

This result by a new PHP code used by the extensions.

As you are sharing the same PHP code for all the websites, this mean that you have a new PHP code in the slave site.

When there is an upgrade or a migration, you have to re-install the extension in each slave sites to let the extension doing its specific stuff to proceed with the MySQL table modifications.

on your site is written i have to maintain only 1 master site... so this is only true for joomla itself but not for the extensions?
can i simply install the new version of an extension with the joomla installer in each slave site without affecting the data?

NO.
One joomla means Joomla with all the extensions installed. This mean all the PHP code of the joomla website.

In other words, this mean that you share the same joomla webistes directory and subdirectories. All the files and folders present in joomla (components, modules, plugins, languages, templates, ....)

As you share the same PHP code for all the website, you can NOT install an extension with different version in the slave site.
All the websites use and share the same PHP code and therefore use the same version.