Each CPU has a number of cores and threads. So called hyperthreaded CPU's double the number of threads i.e. jobs that can run concurrent so a hyperthreaded dual core can run 4 jobs concurrently, a quad core HT can run 8 and bigger, where the default for BOINC is to use them all up to a certain limit. Got an 8 core HT, it will/can run 16 jobs at the same time.

The limit can be memory. Having a 16 threaded CPU and only 8GB of memory could cause BOINC at some point to switch jobs into 'waiting for memory' mode, but it's rare.

I would go with CPU. You mention 1,2,4 task running so I presume old machines. Check the specs of the motherboard and get the faster CPU for it. eBay is the place for used CPUs.
Check the CPU ranking at cpubenchmark. This is the URL https[:]//www[.]cpubenchmark[.]net/cpu_list.php
I have no connection to PassMark except as a customer.

I just check for the model number with CRTL-F instead of their engine. Like if I want specs on an AMD Phenom 8250 Triple-Core I just search for 8250. I look at CPU mark.

Then check the price relative to the rank of the all the CPUs for the mobo.

I just did one AMD and went from 2 tasks to 4 tasks. About $30. Awaiting an Intel iCore5 CPU which will boost another machine from 4 to 16 for about the same price. I finally found a use for all these pieces of junk I have.

I wouldn't bother with a laptop.

Yes memory may come into play but CPU is first.