How to check how many Values are equal to true within all Players?

Data structures

A table is defined as a data structure for a reason - to structure your data! How you organize and store information has a big impact on efficiency, and readability. To spare you a lecture, you should use a table to store the information you're trying to access. Specifically, a dictionary. In short, a dictionary allows you to store information with variables, inside a table. This of course would be opposed to storing information inside a table, with no variables, like this: {"string", true, 123} whereas a dictionary would look something like this: {a = "string", b = true, c = 123}.

Organize

A great way to keep track of your player's data, is by using the player itself as a key in your dictionary table. For example player_data[player] = value, where player_data is your dictionary and player is the key associated with value in that dictionary. A good time to create such data, would be when the player joins and when the player leaves. This is what can also be referred to as session data, since it's temporary, and will disappear when the session (your game instance) ends. Here's an example:

-- Game services
local Players = game:GetService("Players")

-- Session data
local SessionData = {}

local function PlayerJoined(player)
    -- When player joins, use them as a new key in the table.
    -- Their key will be associated with a new table, holding their information.
    SessionData[player] = {}
end

local function PlayerLeft(player)
    -- When the player leaves, remove them and their data from the table.
    SessionData[player] = nil
end

Players.PlayerAdded:Connect(PlayerJoined)
Players.PlayerLeft:Connect(PlayerLeft)

This is a great start. Now you can look for an individual player's data, or all player data with ease! Let's try using this example in your scenario:

-- Game services
local Players = game:GetService("Players")

-- Session data
local SessionData = {}

-- Function to get a list of all players who have Playing set to true
local function GetPlayersPlaying()
    local playing = {} -- list of playing players

    -- Search through session data for players 
    for player, playerData in next, SessionData do
        if playerData.Playing then -- If Playing exists, or is true, then...
            playing[#playing + 1] = player -- Insert player in list
            print(player.Name.." is playing!")
        else
            print(player.Name.."is not playing")
        end
    end
    return playing -- return list
end

local function PlayerJoined(player)
    -- When player joins, use them as a new key in the table.
    -- Their key will be associated with a new table, holding their information.
    SessionData[player] = {
        Playing = true; -- Hold a boolean that represents 'Playing'
    }
end

local function PlayerLeft(player)
    -- When the player leaves, remove them and their data from the table.
    SessionData[player] = nil
end

Players.PlayerAdded:Connect(PlayerJoined)
Players.PlayerLeft:Connect(PlayerLeft)

Now you have a nice, organized way to get player information within a session. You can just call GetPlayersPlaying and it will return a list of players who have Playing set to true in their session data. Hope this helped, if you have any questions, let me know.

Edit

A less-documented, blatant representation of everything optimized to handle what you're asking about, would look something like this:

local Players = game:GetService("Players")
local PlayersPlaying = {}

local function GetPlayersPlaying()
    local playing = {}
    for player, isPlaying in next, SessionData do
        if isPlaying then
            playing[#playing + 1] = player
        end
    end
    return playing
end

-- Function that can set a player's playing state
local function SetPlaying(player, playing)
    PlayersPlaying[player] = playing
end

local function PlayerJoined(player)
    SetPlaying(player, true) -- doesn't have to be true right away
end

local function PlayerLeft(player)
    PlayersPlaying[player] = nil
end

Players.PlayerAdded:Connect(PlayerJoined)
Players.PlayerLeft:Connect(PlayerLeft)

Same concept, you can use GetPlayersPlaying to return a list of the players who are indeed playing, and SetPlaying to set the playing state (true or false) of a player.