When adding new ITE circuit breakers or other brands, you may come to the limit of what your breaker box can physically hold. Some people are tempted at this stage to add mini circuit breakers, but before doing this you should read the building code for your home, apartment or office to see what is allowed. You may not be able to legally install mini circuit breakers except as a temporary measure, and in these cases your building may not pass an inspection until the mini circuit breakers are removed.
In situations like these it is better to install a sub panel next to the main breaker box and run the circuit from the sub panel instead. If you don't know how to do this, it's a job better left for a qualified electrician, but if you are a do-it-yourselfer you can purchase the sub panel and any necessary ITE breakers and get the job done after a bit of research into what your building code may require to do the installation legally. Once you have the sub panel installed, it's a very good idea to have an electrician do an inspection of your electrical system to make sure everything is up to code.