Depends on the foundry.

Pro is not a standard but a commercial designation used to identify fonts with additional glyphs and features supported by OpenType format. Initially, Adobe introduced this term to distinguish their renewed OT fonts from the standard ones (which are just Type1 fonts converted to OT format). Quickly, most foundries began to use this word to promote their products, with a variety of criteria (or the absence of any).

Usually, a Pro font includes wider language support, sometimes with more than one script (Latin, Cyrillic and Greek is a common set up). These fonts often include small caps, oldstyle and tabular figures. Some also offer swashes, additional symbols, fractions and some Mathmatical signs. A few ones are more complete, with alternate glyphs, language-aware substitutions, super and subscript, technical symbols, dingbats, flourishes, combining diacritical marks, etc.

The number of glyphs is not a criteria to identify a Pro font, but of course fonts with larger sets of features would have hundreds or even thousands of glyphs. Finally, please note that some foundries and designers simply do not use the Pro term altough their fonts are very complete (see, for example, recent Windows or SIL fonts, which are not Pro but include thousands of glyphs).

Pro is not a standard but a commercial designation ...

I was pretty amazed to encounter an orange juice brand name bearing the suffix "Pro".

PRO Orange Juice… more of all those diacritical vitamins I guess.

Here are some older threads on the subject:
Is "PRO" really "PRO"
FF Meta OT or Pro? What's the difference
Is the term "Pro" Adobe property or a defacto standard
OpenType, TrueType, Type One PostScript
(Thanks to Florian for previously compiling some of those)

@Freiberger thanks for breaking that down. I guess I can accept the nebulous standard —I was expecting sort of black and white guidelines.

@nina thanks for the links. I did multiple searches but couldn't get decent results. thanks.