Mizzix of the Izmagnus has an ability that gives you an experience counter as long as you cast an instant or sorcery that is of a higher cost. If I have 5 copies of him and 4 experience counters, and I play an instant that has a converted mana cost of 5, will I get 5 experience counters as they all trigger at the same time?
You get 5 experience counters, because the "with converted mana cost greater than the number of experience counters you have" is checked only when determining if the ability triggers. It is not checked when the ability resolves.
603.2. Whenever a game event or game state matches a triggered ability’s trigger event, that ability automatically triggers. The ability doesn’t do anything at this point.
603.3. Once an ability has triggered, its controller puts it on the stack as an object that’s not a card the next time a player would receive priority.
603.3b If multiple abilities have triggered since the last time a player received priority, each player, in APNAP order, puts triggered abilities they control on the stack in any order they choose.
Note that this would work differently if the ability were worded
Whenever you cast an instant or sorcery spell, if its converted mana cost is greater than the number of experience counters you have, you get an experience counter.
In that situation, the ability has an "intervening 'if' clause", which would cause the "if" condition to be checked again when the ability resolves, and the "if" would only be true for the first trigger to resolve; it would fail for the rest.
603.4. A triggered ability may read “When/Whenever/At [trigger event], if [condition], [effect].” When the trigger event occurs, the ability checks whether the stated condition is true. The ability triggers only if it is; otherwise it does nothing. If the ability triggers, it checks the stated condition again as it resolves. If the condition isn’t true at that time, the ability is removed from the stack and does nothing. Note that this mirrors the check for legal targets. This rule is referred to as the “intervening ‘if’ clause” rule. (The word “if” has only its normal English meaning anywhere else in the text of a card; this rule only applies to an “if” that immediately follows a trigger condition.)
Only mentioning this to point out that Mizzix does not have an intervening "if" clause, and thus it doesn't matter how many experience counters you have when each ability resolves.
You do get 5 experience counters.
They trigger at the same time, and all go on the stack, and resolve one at a time.