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I think you would need to test one and see what the result is from various locations. Much depends on the materials used in the construction, and even with a detailed plan I doubt that anyone like to give guarantees.

If those tests look promising, then you can have many emonTH's transmitting to a single receiver. But you can only have 30 if you use our sketches because of a limitation in the JeeLib library that we use (and you'd need to reprogram each with its individual node ID). To get more into the system, you would need to use another radio band and a second receiver, or heavily modify the library. For all those that use the same frequency band, the problem will be two transmitting at once. The probability of that happening is not zero, but as the message is short and if they only transmit once every (say) 5 - 10 minutes - which should be reasonable as temperature normally changes only slowly - then the number of lost measurements should be acceptably small.

It's a wooden structure, is it possible to amplify the receiver with a larger antenna?

Also, i'm not going to be able to connect the rpi to an outside internet connection, only internal fiber, is it possible to avoid Emoncms and run a webserver on the pi for local access for the presentation?

If the building is wood, then I think you stand a much better chance, but I'd still recommend a test. Other aerials have been tried (a search might turn up something) but I can't remember the detailed results.

Don't confuse emonCMS and emoncms.org. We have a (slightly) stripped-down version of emonCMS that runs on a Raspberry Pi. That's what this thread in the list on the right is all about: "(Release Candidate #1 ): emonSD-20Feb16 - emonPi / emonBase (Raspberry Pi + RFM69Pi) Emoncms v9 pre-built SD card image". You can add a hard disk drive to your Pi if you don't want the restrictions of the SD card low-write version. If you don't want emonCMS at all, you'll need to arrange your own database to receive the data and your own reporting and display mechanisms. 

is it possible to amplify the receiver with a larger antenna?

In effect, it's possible to amplify the signal with an antenna. (both received and transmitted)

Some things to be aware of:

1 - Good antennas aren't cheap.
2 - Connecting them to the radio module isn't difficult, but it must be done properly, or receiver and transmitter performance will suffer.
3 - Use good quality RF connectors and feedline. Keep the feedline as short as possible.
4 - Mount the antenna as high as possible and clear of obstructions. e.g. trees and buildings.
5 - The effective gain comes at a cost. The more gain an antenna has, the more directional it is.
6 - If you locate the antenna in the middle of your complex, you'll want to use an omnidirectional antenna, e.g. a monopole, not a Yagi. I f the antenna will be located at one end of the complex, then a Yagi should be OK.

Here's what forum user JD did to solve a range issue:
https://openenergymonitor.org/emon/node/10215#comment-28121

You may not need the same antenna JD used. (made by Arrow, if memory serves)  Any decent quality UHF Yagi or Monopole antenna with sufficient gain, e.g. those made by Cushcraft, Diamond, Hy-Gain, Larsen et al, should do the job.

Can emonCMS be run entirely internaly with access to the web portal without an outside internet connection? Where can i read more about this?

"Where can i read more about this?"
Try the black menu bar at the top of this screen.