The minimum goal is 5€. There are no limits for the maximum amount you can ask!
The more you ask, the more it will be difficult to reach your target as difficulty increase with the requested amount. This will require more work on the project and its promotion.
If you have a community strong enough, or good channels to share your project with, that’s great. But if you are lacking one of these elements from the beginning, it’s best to be realistic on the amount you will ask. Ulule is based on the “All or nothing” concept. This means you can always end up with more money than your goal... but never less. It’s much better to aim for a minimum budget and work hard to reach and surpass it than aiming a target too high and missing it.