Two maximum, but if possible only one. I hate the idea of a partially completed project that's sitting on the shelf gathering dust whilst waiting for either a) an essential part or b) another piece of equipment to be finished. So, I have a series of large boxes where parts for each particular project are accumulated and once everything is available I build the project. A little bit of project management is also involved because one project may depend on another being complete, for example, it would be silly to build the new frequency counter until the frequency standard is up and running.
Wow, such self control! But it sounds like you rarely begin projects where there's a large element of "I don't know how to do this ... yet" and so they never get stuck while you try to work out some (or multiple) difficult problems?
This would be my #1 project-stall cause. Sometimes it's very unexpected - for instance I have a whole building (large shed for metal casting, woodworking, and a blacksmith's forge setup) completely stalled due to a stupid problem with fabricating large (and odd) stainless steel hinges for the main doors. Did not expect to have a problem with that. And once a project gets bumped down the queue for reasons like that, it's hard to motivate and find time to bump it back to the top again. Ha ha... but 'sitting on the shelf' is not a problem with that one. It's a building. It
contains shelves. The only thing it accumulates is leaves on the roof, and piles of junk inside since as it is now I use it for random storage. Oh, and possum nests, since, absent doors.
Also I find interdependencies are the next biggest stall cause. Especially when combined with 'don't know how to do it'. For eg some custom rack-mounted gear is stalled because I want the instrument fascias to have color anodized artwork, and so I need both an anodizing setup (easy) plus facility to ink-jet print onto metal surfaces (anodized but not yet sealed.) Would also like to be able to ink-jet print onto curved metal surfaces, eg cones of dials. This project is hard, and requires a whole 'nother series of projects.
Third biggest problem: time and motivation. Both generally in short supply.