In the other question you linked, you will note that there is a discernible drop-off in your gain per worker that occurs once you have more than 16 workers on minerals and 2 on a geyser. This means you can have 20 workers on a base before you start seeing diminishing returns.
The reasoning for going beyond 20 at any particular base is usually to start massing up for your next expansion. By the time you reach 30 workers on one base, you should be in the process of expanding. After establishing the expansion, you would move workers in excess of 20 to the new base and repeat as necessary for future expansions.
Keeping 20 at each base after expansion also helps you avoid the problem of having many workers bunching up on only a few remaining mineral nodes, especially if you're bad at catching this and transferring workers.
As for the cap, if you're following the above strategy, you'll find that you typically have 3 bases actively harvesting at any given time with a comfortable number of workers for each base, which would put you at around 60 workers. Full saturation of three bases puts you at 90 workers. Any more than that is typically accepted as overkill. This means that, depending on play style and the stage of the game, you'll probably have between 60-90 workers on 3 bases if you are playing at peak efficiency in terms of workers. From there, you'll keep moving workers off of old expansions to new ones until you run out of expansions, then throw them all at the enemy in a last-ditch "crap we ran out of resource nodes" attack.
The mineral mining article on the Team Liquid wiki goes into detail about workers and mining minerals. For some quick information about the maximum number of workers that makes sense, check out their article on resources.
