I have 8 SQL Server installations (on 8 separate servers).
I want a way in which I can estimate future disk space requirements. Can anyone list down the parameters which can be helpful in making such reports?
There is no generic answer to that. Simply said - it depends on the databases. what is also part is not SIZE but more often IO load - IOPS. Especially with large cheap discs you are going to run into an IO limit faster than you can say "damn".
You can see the data for each table with sp_spaceused. Build a script that goes over your databases (sp_msforeachtable will help you here) and collate the statistics.
Take data at weekly or monthly intervals, and plot a trend line (you can even do this with Excel). This will give you a rough forecast that you can use to estimate your future disk requirements. Keep an eye on it and re-evaluate your estimates if your forecast value changes significantly.
Note that linear regression (the most likely method used to calculate the trends) assumes a more or less linear growth. If you have reason to think that your disk usage will grow exponentially you may want to use a more clever forecasting technique.
Depending on how loaded your servers are, you may also need to keep an eye on load statistics like Page I/O latch wait times.