Its been a while since I've been using Alfresco Community Edition 5.1, and I always think about the risks of the application in the next few years, here are my questions: (Please, feel free to comment/answer my questions)
1. How many is the maximum users Alfresco application can take? (current users @ 100)
2. Do I need to increase the server RAM? As the number of users increase and use the Application simultaneously, how much RAM should I ideally use? (current RAM @ 16 GB)
3. Does File uploading, Folder creation, User creation, and etc is infinite depending on the memory capacity you have on your server?
There is no technical limit on the number of users that can be dealt with. The only limit is what your environment supports in terms of performance and the efficiency of any customisations (badly written code may restrict the number of users you can handle).
RAM is only a limiting factor if customisations / addons are written poorly / inefficiently. 16 GiB should get you quite far to begin with. I had customers using 16 GiB servers for up to 70.000 users (500 concurrent).
File uploading is only limited by the content storage available, and this can always be expanded. Also, the size of the database will grow and there must be enough room / resources on the database server to handle that.