How much cayenne powder contributes the same spiciness as one minced habanero?

Notwithstanding the two current answers - that it's really hard to guess - there's an additional concern.

In terms of actual flavour rather than simple heat, you can taste (& smell) habanero in pretty much anything, even at low concentrations. It's a fabulous aromatic. Cayenne, on the other hand, is really almost flavourless in comparison.

So, as well as re-balancing your heat, you need to work on how much of that habanero flavour you want in the final product.

I'd start by dropping your entire chilli content to just one or two habaneros, no cayenne. Taste halfway through the simmer. You can add more of either habanero or cayenne at any time, the flavour of chilli doesn't really change too much over the cooking time that you can't just slip a bit more in if it's too mild.
Work towards 'just enough habanero flavour' & if it's too mild, work in some cayenne nearer the end.

The question is interesting, but it is not answerable. You have quite a few levels on which you are getting a variation.

• The amount of pepper. "One habanero" is not a well-calibrated unit, they come in differnt sizes. Volumetric measurement of powders like cayenne pepper also doesn't give you comparability, you would have to use weight. But even then, depending on how well dried the powder is, one gram of it will correspond to more or less grams of raw pepper.
• The amount of capsaicin. This is the worst problem here - capsaicin content of peppers varies for each crop, because of genetic lineage, cross-polination, growing condiions, and the nondeterministic nature of biological processes. So you cannot say that you have X amount of capsaicin per gram of raw habanero or cayenne.
• The nonlinear human reaction to capsaicin - if you use twice the amount of capsaicin, you won't perceive it as twice as spicy.

Even if we are talking in very broad terms, you cannot make the assumption that one of the two is so "weak" that you only have to change the other. Each of the two contributed. So you will just have to experiment with the curry and see what amount of spice you enjoy most - and don't forget to taste each batch while making it. I would suggest to start from much lower levels, just based on what typical amounts are - maybe one teaspoon of cayenne for a 2-3 liter pot of curry would be a good starting point for a Western cook. But of course, you will want more if you are accustomed to very spicy food.

You don't mention how much curry you made, but that sounds like an excessive amount of pepper for most applications. Cayenne powder ranges from 30,000 to 50,000 Scoville heat units, making it a "medium to hot" pepper. Habanero peppers can range from 100,000 to 350,000 Scoville heat units. These are among the hottest peppers. So your habanero peppers are likely to be at least twice as hot as the cayenne.

Personally, I'd skip on the Cayenne powder. To match the pungency of the Habaneros, you'd need quite a bit of powder. Habaneros have both an intense aroma and quite high spiciness. If you are making the curry for someone who actually enjoys that kind of spiciness, he will be able to enjoy the taste independently of the spiciness, and this amount of Cayenne powder just tastes, well, powdery. It's no fun.

So just use the habaneros. If you find that the result is too hot when the habanero aroma is just right, remove part of all of the orange pith around the seeds (and remove the seeds anyway). That part is by far the hottest in an habanero. Personally, I consider removing it a sin, but tastes differ.