Question 1: Can L2J handle as many client connections as L2OFF ?
Sure. It is elementar and well known by any real programmer that the number of connections can only be limited by the OS, hardware resources or on purpose(source code setup); never by a programming language and or programming platform.
Lineage does use the TCP/IP protocol.
Microsoft Server OS can handle a theoric number of 16,777,214 concurrent TCP/IP socket connections, however it is by default configured to handle no more than 3977 for an unique IP address, if you reach that number Windows will still handle more conenctions at the cost of poor performance but it can be configured in the OS registry to handle more by default, doing so you won't have any OS related performance issues.
Note: On latest MS OS default number has increased.
The same is valid for Linux OS, except the numbers, Linux can handle by default about 11,000 connections, however it can be configured for more.Tt will need changes in config files as well in the core kernel.
The more live connections, more hardware resources are needed.
Question 2: Can L2J handle the same number of clients as L2OFF with the same perfomance?
Sure, no restrictions, no lag, nothing. 'WTF! Noob!'.In fact L2OFF has a much better performance to hardware ratio than L2J, as well C++ to Java.It is just a matter of adapting to the necessary hardware; processor and RAM, mostly RAM.For better performance try increasing RAM before anything.There is no way to find out the optimal hardware for your server other than experimenting.
L2J being limited is a myth.