There are four major verbs in Russian which have entirely irregular conjugations:

Prefixed versions of these verbs and many verbs derived from them are comjugated in the same irregular ways. Ex: выесть, обдать

The above verbs are also important because they're so common. As for other irregular verbs, it's difficult to quantify them all. If you are unsure about a verb's conjugation, look it up in a dictionary (or an online dictionary like Wiktionary- an awesome source!). Don't let the exceptions to the rules intimidate you- you will naturally recall more and more of these irregular conjugations as you learn Russian :)