Most golf clubs are stamped with a number as a means of identification.
Although experienced golfers are often able to differentiate between their clubs simply by viewing their size, shape and loft of their clubface, the number helps make this distinction certain. For beginner golfers, having the number is helpful.
Numbers
The numbers stamped or marked on the heads of golf clubs relate to the club's loft. Fairway woods and irons are the clubs that are always stamped, although specialty clubs also carry some form of identification. A 4-iron, for example, has less loft than a 9-iron. In the same way, a 3-wood has less loft than a 7-wood.
Location
In most cases, the number displayed on a fairway wood or iron is always in the same place. On fairway woods, this number is often on the bottom of the club so that it's visible when the club is in your bag. Numbers stamped on irons are usually found on the bottom of the club for the same reason, but some manufacturers display this information on the rear of the club.
Club Covers
Most golfers use clubhead covers to protect their driver and fairway woods from damage sustained in their golf bag. These covers are always marked with corresponding numbers so that even when a clubhead is covered, you can still identify the club with ease.
Exceptions
Golf clubs such as the driver and putter do not typically carry numbers as a form of identification in the traditional sense.
A driver, however, usually has a number such as 9.5 or 10.5 to indicate the loft of the club. Drivers are available in a variety of lofts. Wedges typically don't have numbers.
Instead that have letters such as P or PW for pitching wedge and S for sand wedge. The sand wedge, gap wedge and lob wedge might also be identified by the degrees of loft. The sand wedge, for example, might say "56" for 56 degrees of loft, rather and "S." Putters also are not marked with a number because of its unique shape and flat face.