As few as your follow-up genuinely requires. For most service businesses that's name, one contact method, and a message box. Every added field taxes completion; classic form research consistently shows shorter forms lift conversion, often substantially.
The exception is deliberate qualification: if unqualified leads waste your time, one filtering question (project type, timeline, budget band) is worth its cost in volume. That's a choice about lead quality, not form design fashion.
Two silent killers to fix regardless: forms that fail on phones, and forms nobody answers quickly. Speed of response routinely matters more than the form itself. The first responder wins an outsized share of jobs.