Is there any way to knew what task of any merc i have? I want to knew what merc to take in party to gain quest in mystery point when i got "Convice a Mercerary to visit your village" Like Natalie task 17 and Vol'jin task 16 in screenshot uploaded.
Unfortunately there isn't a way to check. Only way is to keep a spreadsheet and write down what levels you are at when you get a Mysterious Stranger or daily refresh (you could guestimate how far each merc is based on the unlocked equipment though).
That said, it's fairly quick to complete all tasks for your standard comp and then force specific mercs.
There isn't outside of tracking it yourself with like a spreadsheet.
Assuming you have free campfire slots, if everyone in your party has already completed Task 18 (or is otherwise currently on a task), you'll just get a task for a random merc that hasn't instead if you encounter the stranger.
Firestone can track it for you. In the Mercenaries Progression tab, the Completed Tasks column displays just that. However, from my experience, it seems that it just remembers whenever you receive or complete a task for this mercenary while the app is running. Before it sees a merc at your campfire or whenever you abandon a task, it will only display question marks for this merc.