This is one of the more interesting words in Greek, partly because it is so hard to translate.
BDAG says μυριάς can mean, "a group/collective of 10,000, myriad," but it can also mean, "a very large number, not precisely defined, pl. myriads."
It is this second meaning that is most interesting. Basically, it means a "gajillion." Or perhaps a "bajillion." What slang do you use? A gajillion means lots and lots and lots, with no specific number in view.