You can have as many on TEXT events as you want to as long as you pay attention to how you have them set up so they don't cancel each other out.
What happens is that in a single script file, mIRC will only do the first matching event and will ignore any other events of the same type that are beneath that. That isn't quite the best way to put it, but I can't think of another way, so here are a few quick examples:
on *:text:*:#channel: {
do this
}
on *:text:hello:#channel: {
do that
}
If someone says hello in #channel, the first event will trigger because it triggers on ALL text (*). Because that one triggered, the second one will not trigger. If you put the second event above the first event, then the hello one will trigger and the ALL (*) one will not trigger.
Or:
on *:text:*hi*:#channel: {
do this
}
on *:text:*this*:#channel: {
do that
}
With this one, the *hi* will trigger when someone says "this", so the second one will not trigger.
As long as you put them in the correct order, you won't have problems and you can have as many events as you want. The better way, imo, is to use the method I showed above. The reason I think it's better is because (1) it's shorter, and (2) it's much less likely to fail you because of an incorrect order. You can easily see the order it will follow. It will first check the first IF. If that's true, it will do that and then stop. If not, it will check the ELSEIF that follows it. If it is a match, then it does that and if not, it continues down the list. As long as you use ELSEIF, only the first match in the list of triggers will happen. If you want to have them all trigger if it matches all of them, use IF instead of ELSEIF. So, in the *hi*/*this* example above, you could have both happen if someone said "this" by having both be IF in the combined version that I gave you originally.
If any of that is confusing, let me know.
Basically, all events of the same type that are in the same script file are treated as ELSEIF events. IF the first match is true, do that but don't do any others. ELSEIF the second match is true, do that one but don't do any others, etc.