Note that SBAs are not taken until a player is about to receive priority i. e. not during the resolution of a triggered ability. That means there could be creatures still on the battlfield with zero toughness by the time you count the number of creatures that died this turn.

Two simple ways to fix this:

• Make it a reflexive trigger ("When you do" rather than "Then").
• Split the ability entirely and have the +X/+X effect only once it really matters e. g. "At the beginning of combat during your turn".

Now that I look at it, I notice an issue: If your opponent controls Tatterkite, will the reflexive trigger work? I'd guess, no. I don't know whether that's an issue with you.

I think the trigger would still work, though I admit I am not super-knowledgable with the rules here. Nihiita, Lord of Consumption puts the counter on each other creature. Tatterkite's ability evades Nihiita's, but unless Tatterkite and Nihiita were the only creatures on the battlefield, the reflex should still trigger. I would also think, with the current wording, so long as a creature gains a counter from Nihiita's ability, the reflex will trigger and still count any creatures that died in other ways (if this is an issue, could wording be changed to "creatures that died this way" instead?).

> Tatterkite's ability evades Nihiita's, but unless Tatterkite and Nihiita were the only creatures on the battlefield, the reflex should still trigger.

How come? You failed to put a counter on each other creature since there is one other counter you didn't put a counter on. I think you have to write out what you want the condition to be rather than write "do" e. g. (for what you seem to want) "When you put one or more counters creatures this way".

If nothing else, it removes ambiguity.