theres less than 1000 btc2 for sale on the exchange, so if someone wanted to setup another masternode it wouldnt be possible for that person to buy the required coins.
Not that much less than 1000 BTC2 right now, some days there were more. There's new bids and asks every now and then so even if someone can't get it all at one go, people can leave their offer there.
is there an optimal number of masternodes required for a healthy network? are 17 masternodes enough?
Always operational, with plenty of backup nodes if some go down? Yes.
Optimal? No.
The more the merrier. The number is growing as is.
After advertising and/or another exchange listing, which will lead to a coinmarketcap.com listing, things should pick up speed.