How many unique Pokemon are there?
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If there is less than 250, is there any point to having more than 1 of any given Pokemon?
Candy
In order to power up your Pokemon and evolve, you need candy. Candy is one of two main resources involved with powering up, evolving, and gaining a secondary charge move.
Catching provides candy based on the evolution stage:
- 3 candy for base stage
- 5 candy for second stage
- 10 candy for third stage
Raiding
Since the release of Pokemon Go, raiding has become one of the major aspect of the games. While batting the raid boss, having multiple of the same Pokemon can provide a boost when it comes to exploiting type advantage (e.g. 6 Aura-Sphere Lucarios vs Tyranitar)
PvP
Pokemon Go has PvP battle leagues, all of which have different thresholds of allowed CP. There are many top-meta Pokemon, such as Registeel and Swampert, which perform well in various leagues. In PvP, one benefit to having multiple of the same Pokemon is to have a variety of CP ranges to have one in each league.
The Silph Road has done quite a bit of research into spawning mechanisms and their findings have been corroborated by much of the community. It isn't necessarily guaranteed to be 100% accurate but the evidence supporting it is overwhelming. In either case, all their work is reproducible and peer-reviewed so it's largely considered to be the most accurate non-official information available.
From what I understand of their findings, the world map is littered with "spawn points" (marked by the rustling grass animations), which spawn one pokemon at regular intervals. Most spawn points trigger once per hour but I've heard of some spawning as often as every half hour (although it is possible that the location simply has two spawn points very close together). A pokemon spawned in this way lasts for 15 minutes before despawning and will be available to any player who is in range during that time. Every spawn location has different timings so one might trigger at :02 past the hour and another might trigger at :29.
I have not yet seen any evidence to suggest that these points may cease spawning at any time for any reason. We do know that they are tied to locations that historically have increased mobile data usage, which is why there are tons in the downtowns of larger cities but very few, if any, out in the country.
Assuming this is all correct, it is theoretically possible to momentarily deplete an area of pokemon by catching everything that is currently spawned but that will only affect you and will only last until the next spawn point triggers.
If I were to speculate, I would say that the "dry area" effect is a description of the 45 minutes per hour that there are no pokemon at a given spawn point. If your area only has one or two spawn points within radar range (especially if they all trigger at roughly the same time), the effect would be highly visible, which is likely what that other user was seeing. I don't know whether the map turning grey is related to this but I would suspect it isn't. Instead, I would chalk that up to a graphical glitch but I don't know that for certain.
Best Answer
There is no in-game method, however it is important to note if you're searching near a body of water you will be more likely to find water type, the same is true for grass type in forest/woods and normal's in large cities.