Which nations? Several of the available options are basically worthless because of how bad the ai is at all things naval. Others will often have difficulty in getting the necessary military access to get to the war because of their natural rivals being in the way so you may have to blow past your dip relations limit to get the access they need.

"bri" forgot the most important concern: Even your allies and supporters are likely to not join a war if they consider it to be "distant". An alliance with Portugal will not help you in a conflict in Scandinavia, at least not in the beginning. Get support from Denmark's adjacent rivals, like England, the Teutons, the Livonians or Brandenburg.

Also, before you start the independence war, check each one of your expected comrades-in-arms for ongoing wars of their own or excessive debt, because those give them negative modifiers for joining wars, too.