My experience has been that the most used tap sizes on my boat are 1/4-20 and 10-24.  But I just used a metric tap on my old Volvo diesel...

You can find sets of 12 to 16 pieces that include taps, dies and handles for 25 to 40 dollars.  Over the years I have added odd sizes as needed.  Over the past 40 years I have seen my tap and die pairs grow to about 50 pieces including both Metric and Imperial sizes.  Some I have only needed to use once.  Like most tools you get what you pay for, there is nothing more frustrating than having a tap break in a partially tapped hole... Buy good quality.

For shallow holes and holes with bottoms to them there are "Bottom Taps" that allow you to get the threads most of the way down into the hole before the tap bottoms out.

Make sure you also get drill bits of the correct size for the holes you intend to tap.

kevin