Simply click on Task Board which is placed between Temple and DP to see list of all available tasks.
There is no limit to the number of active quests, but to prevent abuse of the points calculation, monsters are not duplicated
If you want to hunt in place with many different monsters you should accept more tasks
For example, the Spectres task doesn't include Arachnophobica in the monster list, so you should also accept the Dream Court task to count those kills.
Most valuable items from tasks are tokens which you can spend on our roulette