Basic Strategy

"use strict";

//Intialize a zero-filled array to tally 0-to-24 count occurances
let counts = [];
counts.length = 25;
counts.fill(0);

//Tally results for simulations and display.
for (let i=0, simulations = 1000000; i<simulations; i++) counts[sevenValuesLeft()]++;
console.log(counts);

function sevenValuesLeft() {
    const ranks = 13, suits = 4, decks = 6, sevenRank = 6;
    let shoe = [];

    //Populate the shoe with cards
    for (let h=0;h<decks;h++) {
        for (let i=0; i<suits; i++) {
            for (let j=0; j<ranks; j++) {
                shoe.push(j);
            }
        }
    }

    //Remove random cards until two decks remain.
    for (let i = (decks-2)*suits*ranks; i>0; i--) shoe.splice(Math.floor(shoe.length*Math.random()),1);

    //Return the count of 7's left
    return shoe.reduce( (sevens, card) => { return (card==sevenRank) ? ++sevens : sevens;}, 0);
}

"use strict";

//Intialize a zero-filled array to tally 0-to-32 count occurances
let counts = [];
counts.length = 33;
counts.fill(0);

//Tally results for simulations and display.
for (let i=0, simulations = 10000000; i<simulations; i++) counts[twoRanksLeft()]++;
console.log(counts);

function twoRanksLeft() {
    const ranks = 13, suits = 4, decks = 8, twoRank = 1;
    let shoe = [];
    let singleDeck = [];

    //Populate the shoe with cards
    for (let h=0;h<decks;h++) {
        for (let i=0; i<suits; i++) {
            for (let j=0; j<ranks; j++) {
                shoe.push(j);
            }
        }
    }

    //Remove random cards to create a single deck
    for (let i = suits*ranks; i>0; i--) 
        singleDeck.push((shoe.splice(Math.floor(shoe.length*Math.random()),1))[0]);

    //Return the count of 2's left
    return singleDeck.reduce( (twos, card) => { return (card==twoRank) ? ++twos : twos;}, 0);
}

"use strict";

//Intialize a zero-filled array to tally running count occurances
let counts = [];
counts.length = 241; // -120..120
counts.fill(0);

//Tally results for simulations and display.
for (let i=0, simulations = 10000000; i<simulations; i++) counts[twoDeckRunningCount()+120]++;
console.log(counts.toString());

function twoDeckRunningCount() {
    const ranks = 13, suits = 4, decks = 6;
    let shoe = [];

    //Populate the shoe with cards
    for (let h=0;h<decks;h++) {
        for (let i=0; i<suits; i++) {
            for (let j=0; j<ranks; j++) {
                shoe.push(j);
            }
        }
    }

    //Remove random cards until four decks remain that represent the discards
    for (let i = (decks-4)*suits*ranks; i>0; i--) shoe.splice(Math.floor(shoe.length*Math.random()),1);

    //Return the running count based on the discards
    let runningCount = 
        shoe.reduce( 
            (runningCount, card) => { 
                const ace=0, ten=9, jack=10, queen=11, king=12;
                const two=1, three=2, four=3, five=4, six=5;
                switch (card) {
                    case ace:
                    case ten:
                    case jack:
                    case queen:
                    case king:
                        runningCount--;
                        break;
                    case two:
                    case three:
                    case four:
                    case five:
                    case six:
                        runningCount++;
                        break;
                }
                return runningCount;
            }
            , 0
        );
    return runningCount;
}