По горло: up to your neck
Once upon a time, after the great lexicographer Vladimir Dal ate too many psychedelic mushrooms and went into a dream state, he announced to his charges — the words in his dictionary — that he was organizing a contest to see which words had the most synonyms. Много (a lot) and мало (a little), who already had a testy relationship, decided to compete. When Dal awoke, he had to give the prize to много, because много чудится, а мало сбудется (much is dreamed but little comes true).
It's true. I saw it in a dream after a batch of the same mushrooms. One thing is clear: There are a lot of ways to say много.
You can express много by metaphor: вагон (wagon), горы (mountains), куча (pile), лес (forest), море (sea), масса (mass), река (river, often said of drink), поток (flood), фонтан (fountain), хор (choir) or тьма (a tremendous amount).
Тьма is an interesting word. One of the meanings is darkness, and I first thought that the image was of something stretching out into infinity like the night. As usual, I was wrong. The second, now obsolete, meaning is 10,000 of something, but today it just means a whole lot: На Болотной была тьма народа (There were a gazillion people on Bolotnaya Ploshchad).
If you don't like metaphors, you can go with similes. For example: the lofty как звёзд на небе (like stars in the sky), the exotic как обезьян в джунглях (like monkeys in the jungle), or the down-home как сельдей в бочке (like herring in a barrel). You might also use the odd expression как собак нерезаных (like uncut dogs), which according to some etymologists originally referred to packs of uncastrated dogs around a bitch in heat.
Or go with hyperbole: конца-краю не видно (no end in sight), немерено (countless), неизмеримо (immeasurable), не считано (uncountable), бесконечно (infinite), бесчисленно (innumerable), никаких счётов не хватит (can't be counted), по горло (up to your neck), не пересчитать (endless; literally "can't be counted"), or числа нет (there is no number [for how many]).
How much was there? Сколько душе угодно (as much as your heart desires). Сколько хочешь (as much as you want). Хоть задницей ешь (up to your butt in; literally "you could eat it with your butt [and there would still be more]). Хоть отбавляй (more than you can shake a stick at, literally "you could take some away [and there would still be more]).
Are you sure there were that many? Да ужас сколько! (It took my breath away!) Выше крыши! (Higher than the roof!) Будь здоров! (You bet!) Громадьё! (A slew of them!)
Was there a lot of food at the reception? Столы ломились! (The tables were groaning!) Ешь — не хочу (You could eat your fill and then some). Под завязку! (Up to the brim!) Навалом! (Tons!)
Was there really so much work? Непочатый край! (There's no end in sight!) Не огребёшься! (You'll never dig yourself out!)
Did you get a lot of job candidates? Ты что — отбою нет! (Are you kidding? We couldn't beat them away.)
Was that guy really so skilled? Бог его не обидел талантами (God gave him talent to spare.)
It's true: много — хорошо, а больше — лучше того (A lot is good, but more is better still.) But beware: Много — сытно, мало — честно (With plenty you aren't hungry, but with little you're honest.)