No. of concurrent alerts you can set depends upon the no. of available credits. If the number of available credits is less than 100 you can set upto 5 concurrent alerts and if the number of available credits is more 100 you can set upto 20 concurrent alerts.
For eg. if you have 10 Credits , 5 concurrent Alerts can be set. Meaning, 5 alerts can be active at the same time.