This answer depend on several factors which include technical risk, safety risk and business risk. System with high risk should be tested more than the system where we think failure is not a big problem. While testing do not forget the fact that Exhaustive testing is not possible.
Project is bound by time and budget. By Testing more and more you can not play with time and budget there should be end point.
You should come up with Test approach which decide how much testing is needed for this project considering all the risk factors. You should decide testing efforts based on level of risk.
To reduce efforts of manual testing, automation testing was introduced which help a lot but has its own limitation. Even with automation you can not do exhaustive testing.
From thousand of combination of inputs and modules you need to decide what is most important to test within time and budget. There are several testing techniques which will help you to prepare combination of data to test.
For Banking and Safety application testing efforts are more than even development. For safety application generally testing efforts doubles than for other applications.
From your previous project you can say 1 tester is needed per 4 developer. Unless you are from same domain, your calculation is wrong, Testing efforts differ project to project. Do not take the decision of test efforts needed so quickly, Analyse the problem and project in hand and then decide.
It is always good to assign priority to your test case. Priority help when we have limited amount of time, we can skip low priority test cases when we have limited time.
Do not test the project completely, Test it sufficiently.