Is it legal to hold the mobile cone on the back of the bot? And stack cone on top? And how many cones does it count as?
Welcome to the forum. To answer your questions: Yes, yes, and (as far as possession) zero if the cones are stacked on the mobile goal.
To delve further your question has been answered numerous times in the Official Q & A and it is explained in the Game Manual. The whole goal of this forum is to educate the community and reduce the number of times something has to be explained. You may want to browse these resources first to get up to speed and then ask questions if you need further clarification.
^ That answeres the question. If you are meaning mobile goals, it is perfectly legal to hold them and as there is no limit to how many you can hold. Another thing is that you are allowed to hoard the opposing mobile goals, as long as you don’t pick the opposing mobile goals up. Touching stacked cones won’t be counted towards the possession limit(But the cone you touch and the cones’ above the cone you touch will not be scored). Since you just joined yesterday, it is most preferred that you read the rules and guidelines of the VEX Forum (As well as reading the manual) before you start posting furthermore to ensure a professional and up-to-date environment.
Hopefully this helps 
- [TVA] Delta III