This article should help you get started: Data Stores
I recommend reading that article, but to quickly answer your questions:
There really isn’t any hard limit to how many values you can save per game. However, there are some limits to how fast you can access data and specific limits per data entry (all described in the linked article)
In order to save something a player puts in a textbox, you would first need to send the string to the server (potenitally via a RemoteEvent), then store this value with a data store using the :SetAsync() method.