well for sapience, you need stakers and node operators and coins for the database operations and coins for liquidity on the exchange... 8k is more than 1% say you get 50 nodes... thats 2/3 the current supply. the point of masternodes is to have enough that one person cant just own a few and start de-anonymizing the transactions.
*edit not sure if theres any studies on a reasonable number of nodes to represent a network well. a number thats low enough to incentivize participants to remain node runners, but high enough to make sure the nodes wont be a weak link in the system in any way.
I wouldn't go below four, its really cheap so most everyone has a chance to get in now
What would be the incentives for running one?
In our case, we actually have a future use case for masternodes - neuromining pool operations. There is a lot of development effort between here and there, but it is an actual useful purpose for them.
I think we have to be careful to strike the right balance when it comes to the collateral amount. I've been watching some of the latest batch of coins lately, some are lower and some are higher. It seems that if you make it too high it seems to limit the interest and participation. I need to put together a spreadsheet and run some numbers and see what makes sense.