Does a construction site need an ADA unit for a crew of 12?
Yes, you need an ADA-compliant portable toilet for a crew of 12. The city inspector is correct. OSHA 29 CFR 1926.51 sets minimum toilet numbers for construction sites, but the ADA Standards for Accessible Design require accessibility regardless of crew size or ability.
OSHA minimums
OSHA 1926.51(c) says 20 or fewer workers need one toilet. Your crew of 12 meets that with one standard unit. But OSHA does not override the ADA.
ADA cluster rule
The 2010 ADA Standards require that where portable toilets are provided, at least one must be accessible if multiple units are clustered. Since you have one unit, it's a cluster of one. The rule applies to any temporary toilet facility on a job site accessible to the public or employees.
Enforcement
City inspectors enforce both OSHA and ADA rules. Failing to provide an accessible unit can result in fines and stop-work orders. It doesn't matter if your crew is all able-bodied; the rule is about providing equal access.
Order an ADA-compliant unit. It has a wider door, grab bars, and turning space. Your rental company can swap your standard unit for one. It's a straightforward fix. Any questions on placement or service?