If the boss assigns a target of X sales a month and the team members find it in-achievable. What should be done from both sides?

Well that depends on WHY the target was set in such a way: Maybe your boss is inexperienced (then you can discuss with him and make him see it your way), he could be overeager to please his boss (little less likely you will change his mind), maybe he is given the target himself (for any reason that prevents him from changing HIS BOSS' mind), maybe he's an ass*ole sucking up to hiss boss at the expense of his team (no help there), or maybe he just wants to get rid of you (well, solution to this one is obvious..)

So, which is it? ;)

If the whole team is fully convinced that the target is unrealistic, then it should arrange a pleasant & productive dialogue to address their issues & concern to the manager in a professional manner. The manager & team can discuss & agree on how to handle the issue from different perspectives with a view of arriving at the same page-achieving the best. 

Thanks for the invitation 

1. This is where team work comes to play. You communicate with other team members, share ideas on how the sales could be made. Multi tasking comes in with proper planning and self motivation. Going extra miles , locating new potential customers. And continually seeking support from your boss.