The number of things we can focus on at once In terms of computer science, a programming runtime like JavaScript can be assumed to have 1 thread, or one thing to focus on at a time. The processor moves sequentially through instructions, essentially focusing on one instruction at a time.

If you have multiple processors (like a laptop might have 4 or 8), then theoretically you can focus on 4-8 things at one time.

Likewise, I am wondering how many things the brain can "focus" on at one time simultaneously. That is, how many "processors" it has to process information sequentially. I am not sure if the brain even works similar to this, but wanted to ask anyways just in case.

For example, we might have a list of todos in the "back of our mind", a "few current todos" we are "actively" working on. Then at the same time we are minimally aware of the environment so we can hear surprising noises and react quickly to them (for example), so there is a processor available for emergencies (so to speak). So there is at least 2, the active one and the passive polling one. But if there is our "active" task, then what we have lingering in our thoughts might be a secondary focus/processor, so perhaps there a third processor.
You already seem to be making the difference between a point of focus, versus "things in the back of ones mind". I think that's quite apt.

Different authors have come up with different theories about this subject. I think it's a difficult question but I'll just give my hunch, supported by a (very) few papers that supports that view. Note, that there's a large body of literature on the topic, particularly on Working memory (Burmester, 2017). Working memory is:


  [T]he ability to hold and manipulate information in mind, over brief intervals. 


In an overview article, Oberauer (2013) claims that working memory is, as you already exemplify, often explained in terms of physical metaphors such as information flow or information containers. 

A popular scientific blog gives a nice list of problems with the computer versus brain metaphor, most notably:


  
  Brains are analogue; computers are digital
  The brain uses content-addressable memory (memories lead to recall of related memories, called association)
  The brain is a parallel machine; computers are modular and serial
  Short-term memory is not like RAM
  Unlike computers, processing and memory are performed by the same components in the brain
  


Having said that, in general, it is thought that focused attention can hold up to about four independent chunks, or as little as a single chunk, dependent on the theory as well as on the task at hand. You can imagine that more complex tasks 'consume' more 'processing power'. Indeed, a simple 2-digit recall task seems to rely on the two digits being reproduced in parallel (Oberauer, 2013).

In sentence reproduction during active listening, it seems that attention focus switching is necessary, as subjects need to switch their focus of attention momentarily away from ongoing language processing to memory retrieval. In other words, more complex tasks seem to be more serially controlled (Finney et al., 2014).     

References
- Oberauer, Front Hum Neurosci (2013); 7: 673
- Burmester, Sci Am, June 2017 issue
- Finney et al., Child Developm Res (2014); 20734