Will the completion time for each WU increase if I run more than one at a time?

Just raising (or lowering) the CPU count to change the number of concurrent jobs won't change the runtime; each job will follow the project's target runtime.
If you change the project target runtime, then yes, it will affect all WUs including those you have already downloaded.

Ok, but will I complete less work? (i.e. less structures in the same runtime)

CPUs with multiple cores and hyperthreading can run multiple WUs at the same time. If that increases the temperature of the CPU then it may throttle the clock rate to avoid exceeding the maximum thermal design power (TDP) of the chip. The result would be fewer models completed per thread in a given time period. If you are running eight WUs at once and throttle back by 15% then you are still getting a lot more work done overall.

I did some tests and turns out if you run more WUs simultaneously the work completed for each WU may significantly decrease. Obviously you'll still get more job done than when you just do one at a time. From what I read on the world community grid forums in the section of the sub-project that uses Rosetta, this pehnomenon is linked to the L3 cache of your CPU.

I have a quad core laptop with hyperthreading and 32 GB memory. By default, BOINC chooses to run 8 work units at a time. I don't care about credits. I only care about promoting the science. Is it better for the project to limit the threads to 4 or do 8 with hyperthreading? If I want to limit to 4, how would I do that?

On a desktop the general rule is that if there's enough RAM, then run one thread or logical core, and you'll get about 30% more throughput, but will use some more power in doing so. But on a laptop you're much more likely to go into thermal throttling, so I'd recommend limiting one thread per real core. There will be less cache contention, so it'll be more efficient.

---

If I want to limit to 4, how would I do that?

I agree with dcdc's advice. If you can do 8 cores without thermal problems, then that produces more output than 4 cores.
But if not, you can limit it to four cores in the BOINC Manager settings. Set it to "use at most 50% of the processors".

---

Thanks, the laptop has an Intel Core i5-8350U CPU. At 4 cores, the CPU was typically at 80% so I set it to 5 cores and it's now very close to 100%.