I have a Permesso di Soggiorno. Do I have to travel with my husband to the Netherlands or can I travel alone?
If you have a residence permit from any of the Schengen countries, you can travel alone to any other Schengen country, subject to the "at most 90 days out of any 180 days" rules.
This is independent of why you have the residence permit.
(Italy and the Netherlands are both Schengen countries, and Permesso di Soggiorno is the Italian name for a residence permit).
Traveling with a spouse can matter in certain other cases, namely if you're going to certain non-Schengen EU member states and your residence permit is issued to a "family member of a Union (or EEA) citizen". This will usually only be the case if your husband is a citizen of an EU/EEA citizen other than Italy.
(The only states this is relevant for may be the UK and Ireland. I think all of the other non-Schengen EU members actually allow visa-free travel for holders of Schengen residence permits in general, as part of aligning their visa policy with Schengen's).