Depending on the bull and the standard the AI company has selected, semen straws can contain 6,000 to 20,000 sperm cells. No shelf life known - but then AI with frozen semen has only been around since the 1930s or 1940s. Not long enough to tell if it really can be stored "forever."
Semen extenders are added by the AI company to keep it from damage during freezing and thawing... glycerol, milk and/or egg yolk, antibiotics, etc.
For curiousity's sake, there's plenty of interesting information about frozen semen from bull to AI tank, but unless you're planning on working for an AI company, all you really need to know is how to handle the straws between pulling them out of the tank and placing them in the cow. If you have an AI tech come and breed your cows, all you really need to know is cost and how to select the right bull for a particular cow.