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Even if we ignore underwater volcanoes, there are still about 1500 active volcanoes on earth. There are at least 20 volcanoes erupting on earth at any given moment.
Now, if all the volcanoes were to erupt at once.
Firstly, the entire atmosphere will be covered with clouds of ash and volcanic gases leading to respiratory problems for many.
Secondly, volcanic ash may lead to clogging of plane turbines and eventual engine failure, resulting in the grounding of all the airplanes.
Thirdly, as sunlight will not be able to penetrate the volcanic ash, entire world will be plunged into darkness.
Lastly, Acid rain formed due to the reaction between volcanic gases and various atmospheric gases will destroy crops worldwide and also lead to Ocean Acidification.

References:
https://www.usgs.gov/faqs/howmanyac...
https://www.forbes.com/sites/robinand...