When I read your post I thought you were referring to the US. I was wondering where anyone could find that many street dogs here in this country.
Yeah, Mexico is just like any other third world country in Latin America. Full of strays.
I just spent some time back in my home country and I was amazed how many dogs were wondering the streets and beaches. They have similar programs down there, but it's hard to keep up when the culture is not to spay/neuter the very own family pets.
It has improved, but far from an acceptable and under control situation.

Spay/neuter would be key in getting the population back down.

Umm... 300,000 stray dogs already sounds like a disastrous problem. I just hope they are aggressive in picking up the strays and fixing them, as well as making sure dogs in homes are also fixed.

"The city has also introduced an ambulance that will offer care to both street dogs and local pets. Unveiled just last week, Ambudog is Mexico’s first ambulance dedicated to the care of animals. Veterinarians with Ambudog are offering free healthcare to the city’s cats and dogs, whether they live in a home or wander the streets. This includes vaccinations and spay/neuter services that will hopefully put an end to pet homelessness in the future."

Cool the county where I live became no kill animal shelters