Alternative to Terminal Server -- 2

Need to develop a software which is an alternative to Microsoft terminal server. Means it should work like Microsoft terminal server or citrix. The reason I need this software is I have a cloud sever in windows 2012 R2. I need about 30 users to work concurrently on the server. I have few options. To buy MS terminal server cals. to buy citrix. to buy third party software which will do the same function. Software should support unlimited number of users concurrently. Should not use any cracking or illegal methodology Software should support windows server 2008 R2, 2012 R2 I will need the source of the projects If you do not understand my requirement please do not quote.