How many external connections

I was wondering how many external connections a Plex Server that was hosted on a Synology DS918+ (with 16gb of ram as opposed to standard if it makes a difference) would support. If it would not support 4 simultaneous connections of 720p - 1080p, what would it take to support that necessary load?

Would a QNAP NAS be better suited, or would the only feasible option be a custom built NAS? The hardware would be ONLY for functioning as a NAS/streaming.

Thanks all!