Adrian,,, I moved your post to the PCP forum,, it'll get more attention that way and it's the proper forum for it to be in. As for how many fills you will get it will depend on the size of the scuba tank you purchase as well as the psi you fill you rifle up to. If you look in the "Library" section you will find a "fill calculator" that, once you enter the appropriate #'s, will tell you exactly how many fills you will get.
If you get a tank that fills to 3000 psi and you want to run your Mrod at 3000 psi then you will only get a couple of full fills out of the tank after which the pressure in the tank will drop below 3000 psi. At that point you'd only be able to fill the gun to the psi that's remaining in the tank, which will be less than 3000 psi. If you want to run your Mrod at 2500 psi then you will get quite a few more fills.
If you get a 4500 psi carbon fiber tank then the # of fills increases dramatically, however so does the price of such a tank.
The "fill calculator" will give you exact figures once you punch in the #'s for different tanks and gun fill pressures.....
Alot of things to consider. My advice is to research all you can, read all you can here and other places online and then make an informed decision. I'm sure some of the other GTA members will chime in and give you excellent advise in this matter.....
Jeff