D&D 5E (2014) effects of dragons becoming more numerous in the world

[PnPgamer]

Also some of the human populace might go insane, start their own cults either siding with the dragons or asking for devils, demons, old ones or something else for help. Some may become cannibalistic.

 

[Derren]

Self correction when food sources become rare, quickenedby increasing numbers, internecine dragon fighting leading to maybe a few remaining massive ones hunting increasingly depleted food sources

If variant dragons are used this might not be an issue thanks to Goodberry.
So with food not being an issue any more dragons go for the next two things. Treasure and power. And for both they need minions. So assuming a sudden appearance of dragons there would be initial clashes over choice picks of territory and maybe also coordinated attacks against potential non draconic enemies (remember, dragons are much weaker in 5E than in previous editions).
After that phase with territory mostly fixed dragons would subjugate the inhabitants of their realms depending on their personality/type. Lawful dragons would try to set up a aristorcratic or theocratic government with them on top. More chaotic ones would rule by intimidation and force. In some areas, for example deserts, blackmailing by controlling water or other critical resources is also possible.

Once they have their territory under control they will resume their fight against each other, but in a much more methodical fashion than simply fights between dragons. Young dragons born after the territory was split up either would have to serve an overlord in the hopes of being grated land on their own (or hoping that there will be an opening where they can take over), explore for unclaimed land or fight to the death with weakened/young dragons over their territory.