Which video conferencing software should I use?

Question

Video broadcast

Good afternoon.
What is:
Video conferences are held on an ongoing basis. We use conf. megafon.ru. The channel is 40 Mbps, if 20+ people connect, the quality sags. Almost all are in LAN, there is a server on windows looking at all subnets. Due to gluttony, conference tabs often fall. Some machines still have 2GB of RAM.
What is required:
Conferences, camera + sound + screen sharing (or showing documents). It is desirable that it would be spinning in the local area.
Please let me know what to look for and try to solve this problem.