Tetanus shots in adults are good for 10 years so if you've had a tetanus shot and it hasn't been 10 years since your last tetanus shot then you don't need another tetanus shot until the 10 years have passed.
Even if you get cut by a rusty piece of metal or step on a rusty nail you don't need another tetanus shot as long as you've already had one.
If not then you need to get a tetanus shot soon after getting a cut with a rusty piece of metal or stepping on a rusty nail.
If you don't get a tetanus shot then you could be infected with tetanus which could be deadly soon after becoming infected.