How much equity should be kept as an emergency fund?

Our financing is already secured
However, I am just wondering, how much own money should one keep in reserve at minimum? Is there a rule of thumb for that?
I would be interested in your opinions.

Apartment from the developer, some buffer for "Sonderausstattung" is included.

I often read here that 10% of the construction costs should be kept as a buffer. After deducting the furniture, we will probably have €35-40K left.

I often read 10% of the construction costs as a buffer here. After deducting the furniture, we are expected to have 35-40K€ left.

Before the start of construction, we didn’t even have that much... for now we only have about 5-10K. However, we are now also doing one or two things outside that were actually planned for later.

Then we are actually quite solidly positioned with 40-45 T€.

we only have about 5-10k for now.

That's fine too. The main thing is not having to provide additional financing. We had previously expected higher costs and had been relatively frugal as well.

If they are really developers, then only the risk of overpriced upgrades and furniture/lamps remains. This is to be assessed differently than building a house with risks for earthworks, etc.