Hi Peter! So you get a certain amount of InMails per month with a paid subscription. The amount varies based on what plan you have (LinkedIn Business Premium vs. Sales Navigator for example). They reset every month, so once you use them, you have to wait until the 1st to get more. You can purchase more of them, but they are expensive ($10 each I believe).
In practice, you will want to focus on sending connection requests to your 2nd level connections and then messaging those once they connect. Since InMails are limited, I would look to use those only for individuals that look to be a really good fit for you and maybe haven’t responded to your connect request…or if they are a 3rd level connection.
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