How many people can be on Skype screen at a time?

Skype

I want to give a speech and would like 5000 people from all over the world to connect at the same time

Realistically speaking, you are lucky if you can get up to 5 people to participate. Anything past that number, you will wind up with a lot of dropped calls. Skype shouldn't really be used for that. Use a teleconference line for this number of people.