There is no hard limit, it depends on your computer hardware resources. Just use the common sense.
There are some common ways to improve performance, such as: disable Visualizer, disable unused plugins, reduce the number of log messages, optimize Name Mapping/Aliases, and so on. Check these links for more tips:
https://support.smartbear.com/testcomplete/docs/working-with/best-practices/tips.html
https://support.smartbear.com/testcomplete/docs/working-with/best-practices/performance-tips.html
Here I don't have any problem with the performance and all about the TC, We have already taking care of all those think,
it is just for curiosity. what is the limit of the project, script unit or keyword test we can add in TC. :)
As stated, Ravik, the only limitation is hardware, really... drive space, memory, and processor capacity. Generally, if a project seems to be suffering from performance, it's a good bet to split it out into multiple projects. I don't know if anyone has done any scope measurements to say, "Given X drive space and Y Memory and with Z Processor, this is how big a project can be." I can't see how such would help because 10 Keyword tests of 50 lines each might act differently than 50 keyword tests of 10 lines each. As mentioned... use common sense and tweak as necessary.
BTW, something else to note... this is one reason why a data driven/table driven/keyword driven framework many times is a better solution. Because, rather than a project consisting of a number of test cases which is going to grow, a project can contain simply libraries of code that would parse external data to execute test cases. The project size is then no longer contingent on how many test cases you have (for that matter, you could TECHNICALLY drop down to, perhaps, one project per AUT) but simply on the amount of libraried code you have.
