This skill assumes you know what to do just that you want to have Alexa keep the count. Just ask Alexa to "start a stretch with a count of 10" or what number you would prefer. Alexa will then say to get into position and start counting. Alexa will then ask you if you would like another by asking you to "continue" or "next".
“Alexa Open Stretch count and start a stretch for a count of 10”