Claptrap bobbleheads are items dropped from claptraps. Unfortunately, the drop rate for this item is extremely low and may require an extended period of time to collect. It varies by luck. The best place to farm for bobbleheads is fighting the MINAC, where there will be endless claptrap spawns until you defeat the MINAC.
Once fighting the MINAC, you will want to take out its turrets to make farming easier. Afterwards, you can do 2 different methods (or combination of both):
1) Active farming. This will require a lot of ammo and different guns. It's best to use the SMG, Assault Rifle and Shotgun. The higher the magazine clip, the better. You simply camp the claptrap spawns and continually shoot into the MINAC where they come out of. This does not increase the drop rate for bobbleheads but you kill more claptraps than method 2 and will typically earn the trophy in at least half the time.
2) Passive farming. This will require a lot of time. All you have to do is find a spot on the map where MINAC and claptraps cannot get to you. Some people prefer the rocks scattered throughout the map but I prefer the small hut. After you found your spot, simply leave the game on for anywhere between 3-10 hours. Afterwards, pick up the parts where MINAC usually stops. For example, right outside the hut is where MINAC will stop sometimes. Once you done this, weaken MINAC until he's almost dead. Wait until he moves to the spot where there are no parts to pick up (outside the hut) and then proceed to kill him. This is important because if you do not pick up the parts where MINAC is killed, you will not be able to collect the parts it is sitting on.
Below is an excellent video detailing both methods. Credit goes to XeoPro.
Other than personally keeping track, there is no way to know how many bobbleheads you have. It is best just to keep farming for until the trophy unlocks.
The choice is yours!
Happy Hunting :)