Answer to the question in the title
"How to increase the number of CPUs in my computer?"
You can either remove your current i7 microprocessor from the motherboard and replace it with one that has more CPUs, or you could connect multiple motherboards together to form an HPC "cluster". You might find the following high-performance-computing threads useful if you're interesting in going in either of those directions:
However, the rest of your question suggests that the title was not chosen so accurately. It seems that you're not trying to increase the number of CPUs in your computer but you're trying to make use of more of the CPUs that already exist in your comptuer.
Nick Papior's comment pointed out that if "8 processors are used" (as your question says), then you're already using all of your processors! This is because the Intel i7 microprocessor does something called hyperthreading which means that each core can have two threads, so 8 physical cores means a total of 16 possible threads.
Is it safe to oversubscribe a computer?
"How can I increase the number CPU used? Is it safe to do that?"
I do not recommend for you to oversubscribe your machine. Nick Papior's above-linked comment mentioned that you can use
--oversubscribe, but it came with a big warning:
"but please test for something simple to see if this actually gives you anything!!!!"
I want to add to that warning, not to just test if it gives you speed-up, but also to test how safe this is: I wouldn't want you to crash your hardware. In this question: Is this "internal error" reproducible when compiling OpenMolcas on other Intel compilers?, I warned users not to even "fully subscribe" (let alone "over subscribe") a personal computer. In that specific case of compiling OpenMolcas, the compilation took about 20 minutes when using 2 cores, but took more than an hour when using 4 cores (which also caused me a big scare because the entire computer froze during that hour, and the fan seemed overworked as if the computer was going to overheat and permanently damage CPU components).
Welcome to the wonderful world of high-performance matter modeling :) It's really nice to have you in this community. Some of my greatest joys have been with doing calculations that gave me experiences like the one you're currently having. At some point we need to accept that we are doing the best we can with the resources that we have available, and we just need to find more resources (e.g. run the calculations on a supercomputer that has 1000s of cores), reduce the computational demands in the project (e.g. don't aim for such high accuracy), or wait longer for the calculations to run.
If you are open to the idea of jumping off the bicycle (8-core CPU) and hopping into a racecar (supercomputer with 1000s of cores), the following threads might be helpful for you (also I have over 1000 CPU years available in total in case you want to try some calculations here):