How many do you think will be required to be a legitimate stablecoin?

You would need way more than 10% or even 30% of the market value of HIVE. But even if the market value of HBD was at 30% of HIVE. It really wouldn't matter until people started using the coin as currency.

The Better Answer

But I will give a better answer to your question.

Stablecoin just means "pegged currency."

The HBD conversion formula is sufficient to create a stablecoin. People can hold as much HBD as they need for their local transactions.

If the witnesses realized that the formula was more important than the number of HBD in existence; then HIVE could do something truly amazing.

HIVE could create "stablecoins" for all of the major currencies. The system could allow users to choice the stablecoin of their choice. People in the EU might choose the Euro, people in Nigeria could use the Naira. People in Venezuela could choose the bolívar.

You might even be able to create stablecoins for silver and gold.

The system simply needs to use a reputable source for prices. The system could use the same three day formula for conversions.

Having a large number of HBD in existence only creates an existential threat to HIVE. A better approach would be to do something cool like creating a conversion process for different currencies.