From this answer, it sounds like Pokémon are able to call for help an unlimited number of times. In addition, it looks like there are some Pokémon that can only be encountered when they respond to other Pokémon's cries for help.
How can I effectively chain an encounter to get Pokémon like Pikachu or Mareanie?
From the same Serebii link as above, there are a few strategies to improve your chance of having a Pokémon call for help:
The first is to simply keep the Pokémon at a low amount of health. A move such as False Swipe will work for this. Here, the Pokémon has a higher chance of calling for help. It won't call for help if it currently has assistance.
Another way is to use the brand new item, the Adrenaline Orb. This item is found in all Pokémon Centers and, when used during a wild battle, will make the likelihood of the Pokémon calling for help even higher. You can only use one per battle, but the effect should last for the duration [of the battle]
Due to Adrenaline Orbs being usable only once per battle, as noted by @UnknownZombie in the comments, using additional Adrenaline Orbs is a very effective way to pass turns during battles where you are trying to chain SOS Pokémon, as you will not be able to use another Adrenaline Orb, but it will use up your turn.
A third way to improve your chaining probability is to use a Pokémon with the ability Intimidate, Unnerve, or Pressure.
The ability of the Pokémon you have in battle is also one that factors into S.O.S. Battles. If your Pokémon has the abilties Intimidate, Pressure or Unnerve then the opposing Pokémon is more likely to call for help.
Thus, the three main strategies are to keep the Pokémon at low health, to use an Adrenaline Orb, and to use a Pokémon with Intimidate, Pressure, or Unnerve.
As an aside:
Do note that even if a Pokémon calls for help, it is not guaranteed that a wild Pokémon will come to its aid.
