I know that the Dow Jones Industrial Average (DJIA) started with 12 industrial companies in 1896, but that's not what I'm asking about here. I'm asking about its contemporary version. The S&P 500 and Russell 3000 do not always contain 500 and 3000 companies respectively. If there is a merger, acquisition, or delisting, they may contain less constituent companies. I'm wondering if this is also the case for the DJIA. Does the DJIA always have 30 constituent companies, or can it vary?
With only 30 companies in the index it is easy to keep it at that number. While these companies do split in two, they do merge with other companies, and they do split their stock price, these tend to not be surprises.
If you look at the changes to the Dow you will see additions and subtractions whenever a split of a component company takes place. They use the time between the potential split, and the actual split to plan their actions. They do the same for mergers.
It would have to take an epic rapid collapse of a company to end up with fewer companies in the Dow. Something like on Friday everybody thinks company X is doing great. But on Monday morning they announce they are liquidating everything.
Also remember indexes like the Russell index are part of a family of indexes. They rank a bunch of companies and then decide where the dividing line should be. They periodically re-sort and then decide if they want to fudge/blur the location of the dividing line a bit to reduce the amount of stocks changing indexes. That leaves them with the Russell 3000 having approximately 3000 companies.