I get nervous between 150 and 200. The real limit is 32k of data besides the
message body.

Just as an addendum to Sue's answer - you may want to consider a few other
things that go beyond simply what Outlook 'allows" depending on what your
ultimately planning to do.

While there is no "practical" limit to the number of fields you can create
(leastwise not in any examples we've run across from customers since
releasing ContactGenie), you may want to remember some of these numbers:

Max # of fields in an Excel worksheet = 255 (or 256 whichever way you work
with things)
Max # of fields in a standard recordset = 255 (SQL DB's aside)

Using a standard contact record - we use the following numbers (the +/-
after the numbers below simply means other people may choose to
include/exclude fields that differ from our list)

Importable fields = 102 +/- (few people use all Outlook contact fields - ie
there are some 19 different phone number fields)
Exportable fields = 113 +/- (ignoring fields like
"CompanyLastFirstSpaceOnly", "YomiCompanyName" etc)
***the numbers above don't take into account importable fields that may be
"multi-part" and as such may span more then one field in your source file
like "Street Address" or phone numbers (country code, area code, phone
number, extension)

We have been sent examples that had some very high numbers in terms of
user-defined fields which becomes a real pain when wanting to import or
export all this along with some of the basic standard fields especially when
the contact records did not include any kind of simple and absolute unique
identifier such as Customer ID etc (EntryID's don't cut it if there are
multiple users trying to keep track of the same info).

Aside from that, to digress from your question for a second,  there are a
number of things that Outlook "allows" that can create problems for you
(such as allowing the creation of custom field names that are not acceptable
database field names - i.e. example - read an Excel worksheet using the
Microsoft Jet Engine and you'll find illegal characters in the column names
automatically replaced which means that you will never get an exact "match"
to its Outlook counterpart (or vice versa) without knowing the precise
mapping structure but this only applies to custom fields which Outlook
itself doesn't handle for import or export).

Just some food for thought as you plan your contact structure(s)......

Karl