I´ve read that you can install APC only if you have one site running in your server. Is that true?

No, it's not true.

You can have multiple sites all sharing the same APC pool. This raises potential security issues, though, so I wouldn't recommend it if you have untrusted scripts (web hosting customers) on your server.

Or you can give each site its own APC pool, using FastCGI. This of course can take up more memory than having a shared pool.

Thanks hybinet.
Can i enable and disable APC anytime? I mean, i want to test a site script with and without APC just to check speed.

Which OS and webserver are you using?

If you're using Debian or Ubuntu and if you installed APC using apt-get, you can disable APC by opening /etc/php5/conf.d/apc.ini in your editor and commenting out the first and only line. (Prepend a semicolon to comment it out.) Then restart Apache.

Thank you! Yes, it´s Ubuntu.

Using an opcode cache like APC can give you a huge speed boost.

Thanks Brian, the problem is that in the administration section of my site i have a few cache possibilities, file based, memcache, xcache and APC, and APC is recommended.
But, thanks i will take a look.