If most of my users are online at a time, qerry time will increase, more system resources will be taken and my system will be very slow thus bad performance.
That would be the same problem in any country if the web host is a small web host that has restricted things like RAM and CPU.
It has nothing to do with bandwidth.
If you want users to admin their own sites, then your choices are multisite or single installs.
In most cases, most of your users will *not* be online all at the same time - unless you're running something like a classroom. In any case, a better web host will help.
You can also install a plugin like hyperdb or shardb to split up the databases and spread the load - but you may still need better hosting for it.
For the first couple hundred users you will not have an issue with this.