Try to avoid confirmig more then one company. But many times it is difficult to practice that. If you confirm and do not join the impact depends on company. Some company's may not entertain you to apply their company atleast for next 6 months to one year and some company's may not have any policy like that. But from your side make sure that you inform the Company by giving enough time to replace you. I have seen cases that people informing the HR on the joining that they are not joining. This very bad because some times company's may have big plans for their new employee and if that guy informs in last minute it is very difficult for them to find new resource.