The reason it fluctuates is Feedburner doesn't count how many people are actually subscribed to your feed, but how many people click read your site via feedburner every day. This means it will fluctuate wildly depending on if you have written a new post or not. If you don't write anything new then it's likely your stats will drop to zero. The numbers that feedburner shows don't really matter anyway.
Okay, I understand now..but then again...Is there any way to find out how many subscribers do we really have?
No, but you can get a pretty good idea. Look at what your feedburner stats show the day after you write a good/popular post. That should give you a pretty good idea of how many you have.
I hope at least 90% of them read my new post on the first day so that I could figure out.. Anyone who can create a site to monitor feed count would be rich!