Hi, Chad! I’d recommend always including the 95 characters in the Basic Latin set and the 96 characters of Latin-1 Supplement at the very bare minimum. With those, your font can generally be used for English, French, Spanish, German, Portuguese, and several other western European languages.
The template I use when creating a new font has just over 520 characters in it – including a bunch of Combining Diacritical Marks, and over 300 accented characters from the Latin Extended-A, Latin Extended-B, and Latin Extended Additional sets. I also include some Currency Symbols, Mathematical Operators, General Punctuation, and a scattering of characters from several other Unicode sets.