On an (even) older Toyota I remember the alternator pivot bolt and nut were 12mm and 13 (or maybe 14 and 13). I figured it was designed this way so a mechanic doesn't need two wrenches with the same size, like 12 and 12. You could do the job with a single set of wrenches.
I think on ours the bolt and nut are the same size. And on some models the pivot bolt threads directly into the bracket, so only one wrench is needed.
16mm I never came across, at least for a bolt/nut that a socket can fit on. Maybe on something like the fuel filter where you can fit a flare wrench on to hold the filter while you loosen the other nut, or maybe on the nuts on the air conditioner freon lines.
I do keep a 13mm socket in my toolbox because at some point a mechanic may have put an ANSI/ISO spec bolt/nut on which is 13mm for 8mm diameter threads rather than 12mm for Japanese spec.
See this:
Metric Bolt Head/Wrench Size
For 8mm thread diameter, Japanese Industrial Standard (JIS) head size is 12mm, but for ANSI/ISO and DIN the head size is 13mm.
Original sizes on our cars should be JIS, so 7, 8, 10, 12, 14, 17, 19, 22; but things like lug nuts could be 21. As more Toyotas started getting built in the USA, they may have started using fasteners with different specs.
See the chart in the link above and compare ANSI/ISO, DIN and JIS.