Dont stress out yourself or do tend to accumulate all of possible bounty programs specially on twitter yet you do have a day job which its obvious that you do only have very minimal extra time.In this case only join several twitter campaigns 1-3 on my part if im busy or do have that kind of situation but if you do have the time or full time then you can join as much as you can. Just put up on list on the task that you had done and not being done to manage it correctly.
I also have a job in real life so I don't have much time to work on bouty campaign, maybe I should take your suggestion and join few bounties at the beginning then I'll join more campaign if I still have more free time to do

. Beside as I can see, Merit point is required for rank increasing, can you suggest me any way to be able to get merit?
New merit system would be somehow hard for you to rank up. Earning merit can be gained by doing good post quality and together with potential activity you would able to rank up. As far as i know ranking up to JR member doesnt really require merit try to read up here
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If you dont have too much time then its better to engage into smaller task than on minding yourself on handling lots of them.