2) As you are obviously new here, I have some suggestions to help us all help you:
When you post, please post your entire configuration including (but not limited to) your installation method (7.X.X?) and vicidial version with build (VERSION: 2.X-XXXx ... BUILD: #####-####).
This IS a requirement for posting along with reading the stickies (at the top of each forum) and the manager's manual (available on EFLO.net, both free and paid versions)
You should also post: Asterisk version, telephony hardware (model number is helpful here), cluster information if you have one, and whether any other software is installed in the box. If your installation method is "manual/from scratch" you must post your operating system with version (and the .iso version from which you installed your original operating system) plus a link to the installation instructions you used. If your installation is "Hosted" list the site name of the host.
If this is a "Cloud" or "Virtual" server, please note the technology involved along with the version of that techology (ie: VMware Server Version 2.0.2). If it is not, merely stating the Motherboard model # and CPU would be helpful.
Similar to This:
Vicibox X.X from .iso | Vicidial X.X.X-XXX Build XXXXXX-XXXX | Asterisk X.X.X | Single Server | No Digium/Sangoma Hardware | No Extra Software After Installation | Intel DG35EC | Core2Quad Q6600
3) This is not a "general discussion", it's a support request. Moving to the Support board accordingly.
4) "have an extension as a trunk" sounds like you are trying to describe something using general terms, but since those are both actually technical terms and have different meanings, we're not really getting what you want.
Extension: This is a number that the PBX will dial in the dialplan. These are Grouped by "Contexts". IE: The Dialplan is made up of Extensions in Contexts.
Trunk: This is generally used to describe a single connection via SIP or IAX2 to another PBX (usually a carrier). Thus in Admin->Servers (select a server)-> Max Trunks: 5 would limit the autodialed trunks on that server (for any carriers) to a total of five.
I'll guess that you want to be able to forward calls to an extension, and you'd like to forward several without terminating the previous calls so that you may have several live calls to the same extension at one time. Guessing.
In this scenario, the limitation is usually in the SIP account connection. If, for instance, the Extension is a phone in Admin->Phones, there is a sip configuration option to limit the number of live channels that sip account is allowed.
This would be accomplished by using "call-limit=4" (or some other number) in the sip.conf entry of the PBX which can be populated in admin->phones (choose phone) and placing call-limit=4 in Conf Override. Be careful, that's not a standard use and other entries may be required. However: If there's already a value in this field, changing that value will change the channel limit.
If this extension is external, and passes through a carrier (in admin->carriers), the call-limit=4 value may be in the carrier's "account settings" field or may be a limitation at the carrier itself outside of Vicidial entirely.