No one species of Pokemon (except legendary Pokemon, and even then) has a set population; hence their numbers are relatively infinite.
If you mean to ask which the most common Pokemon is, then the will vary from game to game. In every Pokemon game, there are a few Pokemon which feature predominately throughout. As the Pokemon generations and regions change from game to game, the most common Kanto Pokemon might not be so common Hoenn and the most common Pokemon in Sinnoh would definitely not be the most common Pokemon in Johto. So let's see which Pokemon are the most common in each region.
The Kanto region is home to an overwhelming amount of Pidgey and Spearow. Spearow is found on 10 of Kanto's routes, while Pidgey is found on an astonishing 14 routes. Pidgey, therefore, is the most common Kanto Pokemon.
In the Johto generation games (which include both the Kanto and Johto regions), Zubat is very common, residing in 16 locations while Ratatta is seen in 22 locations. So Ratatta is the most common in the Johto region.
In Hoenn, Magikarp is the most common Pokemon, being found in all bodies of water (with an Old Rod), amounting to 29 locations. Tentacool comes in second, being found in 20 locations throughout Hoenn.
In the remake version of Pokemon Red and Green (our Blue version), the most common Pokemon shifted, so they must be included here. Tentacool takes over from Pidgey in the original version, appearing in 23 in-game locations. Spearow then comes second, appearing in 16 different places.
Finally, in the Sinnoh region, the most common Pokemon are Wurmple, Silcoon, Cascoon, Combee, Cherubi, Heracross, Burmy and Munchlax who can appear in 19 different locations (as suggested by the 'area' feature on the Sinnoh PokeDex). These would be the locations of the honey trees. While there is a chance of encountering a different Pokemon, they all have close to the same probability, so they all get to be the most common Pokemon. In Sinnoh, Wingull comes in second, being found in 12 different locations.
Overall, though, I'd have to say Magikarp is the most common Pokemon. It is consistently found throughout all the games and is found in every body of water. Magikarp is found by using the Old Rod - which is also readily available in every Pokemon game. So while it isn't the most common in individual games, it is the most common across the series.