How many EVs do the Festival Plaza services give

Questionnaires:

The whisper "Tell me! Tell me!" comprises 16 different requests, all of which are questionnaires the player must answer. Each request has a number of custom responses, as well as a standard "No comment" response. If the player answers "No comment", they will not receive Festival Coins and the request will be exhausted. Any other answer is rewarded. These questionnaires are always asked in order. That is, if two people are whispering "Tell me! Tell me!", whichever person is asked first will ask the next question. After the player answers the last questionnaire, the next requestor will start with the first questionnaire again. The player's responses to these questionnaires appear in their profile when they are visitors to other players' Festival Plazas.

Found on Bulbapedia

Option 1: The 'Reset Berries' are back in this generation, they are acquirable from Berry piles, or from PokePelago. There is a berry for each stat, and they all lower EV by 10 points.

• HP: Pomeg
• Attack: Kelpsy
• Defense: Qualot
• Special Attack: Hondew
• Special Defense: Grepa
• Speed: Tamato

Option 2: New to S&M, you can get 'Reset Meals' from the Festival Plaza. I'm unsure what Plaza level is required for them to start appearing.

Sadly, there is no known equivalent to the 'Reset bag' from XY/ORAS as of yet.