A simple Google search could have given you what you needed. From Wikipedia:
When used for 10/100/1000BASE-T, the maximum allowed length of a Cat 6 cable is 100 meters or 328 feet. This consists of 90 meters (300 ft) of solid "horizontal" cabling between the patch panel and the wall jack, plus 10 meters (33 ft) of stranded patch cable between each jack and the attached device.
When used for 10GBASE-T, Cat 6 cable's maximum length is 55 meters (180 ft) in a favorable alien crosstalk environment, but only 37 meters (121 ft) in a hostile alien crosstalk environment, such as when many cables are bundled together.
YouTube does 1080p at 4Mbps. A Skype HD video call takes up to 1 Mbps for each attendee. Email and other browsing typically do short, high bursts and have very little consistent data transfer.
If all 50 people were watching YouTube videos and doing Skype calls simultaneously it would only take up to 300Mbps. You'll have the other ~2/3rds of that 1Gbit link left for other usage.
I doubt you'll see that kind of traffic, but if you do you'll want to implement Qos.
Since the contractor is already running a line you might as well have him run 2 or 4 lines total. That way if you do ever exceed the limit of that single line you can do link aggregation to increase the throughput. It's also nice to have an extra line just in case one goes bad.
And, yes, you can put a switch in the middle of long runs to extend the connection. Alternatively you could run fibre and use fibre to copper converters at each end if your switches don't have SFPs.