No. Even though you will only have 5 concurrent calls there would still be 900 extensions. But there's more then one way to skin a cat.
If you're dead set on using a BCM, which I wouldn't recommend for this situation (and I design Voice/Data infrastructures for a living), you could use a 50, 200, or 400 in the main site depending on the number of local extensions and trunks you need, and then use a BCM 50 at each remote with one IP, analog, or TDM set (using an analog would save a lot of money but you loose MWI), and one VoIP trunk pointed back to the main site. The main site would need only 5 VoIP trunks since these are shared to all the remotes.
But frankly this situation screams pure IP instead of a hybrid system. What feature are you looking for AA, CCR, CC, VM, UM, these will play a big part in determining if you should be using a simple Asterisk system or a SC1000. Also, what's your IP connectivity between sites.
BTW Like I said I do this for a living so if you're really interested in engaging a consultant you can contact me directly. Otherwise, I look at this group every once in a while.