Only done a few so far Matched, 9.8kWp being the biggest (although quoting for 35 an 40's). so far its been two roofers, (one whos been on a PV course), and myself.
With three someone is available to help inside and out, lifting inverters into lofts, panels onto roof. Two as an absolute minimum, definately not a one man affair. Four might come in usefull when you have a bit more A/C to pull in.