The answer to your question depends on the circumstances for splitting the train into sections in the first place.
In this country, the only trains run in sections generally are passenger trains. If running under timetable authority, a train may have extra sections running on the authority of the lead train. I don't know if timetable authority is used even anymore. In sixteen years of running trains, I've never run on timetable authority.
A long-distance passenger train could be split into several sections enroute to serve various destinations. The best known example of this was the Union Pacific "City of Everywhere". This train had several sections that were switched out or combined enroute.
The size of each section depended on passenger load. Cars were also added or deleted at switching points enroute.
A train might be too long to fit into the passenger terminal platform tracks. Each section would be required to have cars to provide necessary services to the passengers: diners, lounges, baggage cars.
If the train has a large number of passengers, and would be too large to fit terminal tracks, it could be divided into coach and first class sections, to be operated separately. Each section would need its own food service cars and lounges.
Freight trains are not normally operated in sections. A freight train may be broken into two or more parts to get up a hill if there is insufficient power to handle the whole thing in one piece. How this is done depends on the expertise of the conductor. A green conductor may take as much as can be carried up the hill and into a siding, then go back for the rest, even if it is only a few cars. A better conductor will try to split the train in two based on car numbers, so as not to tax the locomotives to the maximum. The best conductors will split the train not on car count, but on tonnage.