When a new client subscribes to the newsletter, will they start receiving it from issue #1 or from the next issue?

A client newly subscribed to the newsletter will receive it starting from the next issue. Past issues are archived on your private Coaches Console website (Newsletter menu), where clients can access them freely. If you want a new client to receive newsletters starting from issue #1, you could create a Custom Autoresponder with the respective newsletters and assign the new client to it.