Is there a limit with Subscription API?

20 subscriptions per user. To avoid creating multiple subscription, you can group them and use the event filters. E.g. if your app needs to receive presence events and telephony session event, you don't need to create 2 separate subscriptions but one with the filters for both events

var eventFilters = [
    "/restapi/v1.0/account/~/presence",
    "/restapi/v1.0/account/~/telephony/sessions"
    ]