Hi Khoa.
 
The maximum number of concurrent TCP connections depend on the computational capacity of the server. Kamailio can be regarded as an application server of high performance SIP/VoIP.
 
In the following post, the Kamailio user set max TCP connections to 64K.
http://lists.sip-router.org/pipermail/sr-users/2013-April/077555.html
 
Actually the theoretical maximum number of open TCP connections is  general setting for all servers (not limited to Kamailio). The following link is a good reference for your question.
"What is the theoretical maximum number of open TCP connections that a modern Linux box can have?" Stack Overflow
 http://stackoverflow.com/questions/2332741/what-is-the-theoretical-maximum-number-of-open-tcp-connections-that-a-modern-lin
 
The max_tcp_connections set to 2048 is a very conservative setting. Theoretically you can set a large number of max_tcp_connections. However, Kamailio node may exhibit overload even collapse, when concurrent TCP connections reaches maximum setting during peak-hour and each connection perform heavy task by average.

To be conservative, you can collect statistics about CPU and memory utilization of Kamailio node under 2048 connections (your current default setting). For example, if CPU utilization is only 10%, you can increase max_tcp_connections setting from 2048 to 20480.
 
Similar to potential overload caused by too many TCP connections, SIP also faces with overload issue recently due to its popularity.
 
Build-in overload mechanism cannot prevent overload effectively. Therefore, IETF SIP Overload Control (soc) Working Group works on IETF RFC "SIP Overload Control" currently.
http://tools.ietf.org/html/draft-ietf-soc-overload-control-12
 
IETF SIP Overload Control discussion archives
http://www.ietf.org/mail-archive/web/sip-overload/current/threads.html#00874
 
A survey on SIP overload control algorithms (including IETF RFC "SIP Overload Control") can be downloaded from the following link free of charge.
Y. Hong, C. Huang, and J. Yan, “A Comparative Study of SIP Overload Control Algorithms,"Network and Traffic Engineering in Emerging Distributed Computing Applications, Edited by J. Abawajy, M. Pathan, M. Rahman, A.K. Pathan, and M.M. Deris, IGI Global, 2012, pp. 1-20.
http://www.researchgate.net/publication/231609451_A_Comparative_Study_of_SIP_Overload_Control_Algorithms
http://arxiv.org/abs/1210.1505
http://www.igi-global.com/chapter/comparative-study-sip-overload-control/67496
 
Control theorectic approaches have been applied to model the interactions between an overloaded SIP server and its upstream servers as a feedback control system in two different scenarios - redundant retransmission ratio control and round trip delay control (IEEE Globecom 2010 and ICC 2011).
 
"Mitigating SIP Overload Using a Control-Theoretic Approach" IEEE Globecom 2010
http://www.researchgate.net/publication/221284946_Mitigating_SIP_Overload_Using_a_Control-Theoretic_Approach
http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=5683124&url=http%3A%2F%2Fieeexplore.ieee.org%2Fxpls%2Fabs_all.jsp%3Farnumber%3D5683124
 
Implementation of implicit SIP overload control in the real system
"An Efficient Earthquake Early Warning Message Delivery Algorithm Using an in Time Control-Theoretic Approach"
http://link.springer.com/chapter/10.1007%2F978-3-642-23641-9_15#
http://www.ipv6.org.tw/docu/elearning8_2011/1010004798p_3-7.pdf
 
"Design Of A PI Rate Controller for Mitigating SIP Overload" IEEE ICC 2011
http://www.researchgate.net/publication/224249824_Design_of_a_PI_Rate_Controller_for_Mitigating_SIP_Overload

Skype story published by Road Runner (TimeWarner Cable Inc.)
http://features.rr.com/article/01vBgtc8TR17z?q=Skype
http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=5963029&url=http%3A%2F%2Fieeexplore.ieee.org%2Fxpls%2Fabs_all.jsp%3Farnumber%3D5963029
 
 
Best regards,
Winston Hong
Software Engineer
InBay Technologies Inc.
Ottawa,
Canada K2K 1Y3
http://www.inbaytech.com/solutions.html