A minimum of 2 trekkers is compulsory for any restricted area in Nepal for issuing special permits. A company and a local staff are a must to do this Mustang trekking in as per the government rule. Special permits cost $500 per trekker for 10 days that’s valid for 10 days. Besides a special permit, you would need an ACAP (Annapurna Conservation Area Project) permit and TIMS trekking permits. However, with Mountain Monarch, it's all set by the time you arrive in Kathmandu.