Welcome to the Party antonynv!
First, newbie suggestions (which will make your stay here much more comfortable):
when you post, please post your entire configuration including (but not limited to) your installation method and vicidial version with 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 "from scratch" you must post your operating system and should also post the .iso version from which you installed your original operating system. 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
Next: There is no "defined" maximum. However, a rule of thumb is anywhere from 150-350 calls per server. No real limit to the number of servers in a Cluster, although it is somewhat cumbersome to exceed eight servers.
That being said, I've had servers up to 600 calls and I've had servers that were only capable of 4 calls (VPS servers usually, but a couple old Pentium IIs had that limit as well).
Do you have a goal? Or is this merely a planning mission? Will your server have 1 processor or 16? Will it be 1Ghz or 3.6Ghz? Will it have 1G RAM or 32G RAM? SSD Drives or 5400 rpm IDE drives? 100Mx100M Bandwidth or 1G x 512K Bandwidth?