I believe it will count as 6 hours, as each droplet will be billed separately.

Quoted from FAQ:
Do I have to pay the cost of the server every time I create a new one?

No — you only need to pay for 1 hour of usage. Spinning up a new 512MB server just to test something for a couple of minutes will only cost you $0.0074

I believe, but don’t hold me down to this you’d be charged 6 x 1 hour as each individual droplet will create a new invoice at the end of the month.