Technical characteristics (limits) of chat in classmates and viber and skype?

Question

Skype

You need to know the max. participants...
Ease of administration...
Ability to log chat text to a file..
Setting up notifications for key phrases (selected participants)
Supported OS... android+windows.... and not bad Linux

Answer 1

SELECT COUNT(*) FROM users
That is, in your opinion, there may be a negative number of users. Oh well.