You can broadcast to messenger and WhatsApp contacts 

I need to do it in Instagram

in Manychats broadcasting shows 3 subcribers. What subscribers is this referring to?

Hey, ​@Malena Lampazzi 

It’s not possible to broadcast using Instagram. These are the channels currently available to do that:

​@wakecreative Those subscribers are the number of contacts that opted-in to the channel you have chosen.
Example: If you collected 3 email address (and they’re opted-in), you’ll be able to send them a broadcast email.