The best way to keep track of how many shots have bee fired is to count them. It shouldn't be hard, since you're unlikely to be training on something that requires extreme attention. If you want to make it easier for your brains, you can use a calculator and dial Ans+1 and on every shot just press Exe or = or whatever it is in your calculator. If your only options are crystal bow or msb+rune arrows, I would probably go for the msb+rune arrows, because you will be able to camp longer, and if you're not willing to count the shots, it'll be cheaper and more time efficient in the long run. I personally would go with the rune crossbow, because it just feels the most comfortable, since there's no need to hazzle with realoading the crystal bows every now and then, and mostly Ava's accumulator collects most of the bolts shot. Compared to msb, if you're training on something that you need defence against, with rune crossbow you can wield a shield, or then just wear a Book of Zamorak. If you've got training to do, why don't you try out the different weapons and compare the hourly experience points gained of each weapon and find the best weapon for you to use.