As you know, a short tapered tongue / leader -- length of film protrudes from the velvet lined mouth of the film cassette. The velvet is a light trap that protects the film in the interior of the cassette from being fogged during the film loading procedure. You would be wise to load and unload in subdued light. In desperation, use your body’s shadow.
We close the camera and advance the film to #1. This indicates that exposure number 1 has been advanced and the camera is ready to take picture no. 1.
We are all tempted to squeeze more shots out of our roll than the number of exposures stated on the box. With luck you can get two or even three more frames.
As a rule of thumb these extra frames should be considered hit-and-miss. In other words don’t deliberately compose and shoot the next Pulitzer Prize image on film that could be despoiled.
Here’s why! The tongue, along with a few subsequent frame lengths, is likely fogged. If you are developing the film yourself, standard practice should be to cut off the tongue and save it for testing fixer and perhaps developer activity.
If you send your film out for processing, the tongue and then some will be used to affix the film to the film transport mechanism of a film processor. This area likely will be used to identify your roll via an adhesive label, or have a number imaged using light or perhaps a number using a labeling punch.
OK to try and get a few more frames than stated on the box. Just be aware, that all of the extra fames might not be fruitful.