Even though there is a spike arrest limit per second, this is further divided into millisecond intervals that you must consider when programming your calls to avoid 429 errors.
As a general rule, if a spike arrest with a quota of X calls per second, the system will convert the second to milliseconds and divide it by the number of calls per second restriction; then, only one call will be able to happen inside the interval of the resulting millisecond-interval.
For example, for a quota of 5,000 calls per day, with a spike arrest of 4 calls in 5 seconds
5 second = 5,000 milliseconds
5,000 milliseconds are divided into four intervals = resulting in four intervals of 1250 milliseconds
Only one call can be triggered every 1250 milliseconds.
If the user tries to do two calls inside one of the 1250-millisecond intervals, it will trigger a 429 error.