In order to stay within our tier of total leads, I would like to set up an email alert that will let me know once we have hit 48,000 known leads in our system. Has anyone tried to do this before? Is there a way to set this up?
No, you cannot do this.
You also do not need to. Marketo only checks your database size during renewal. Unless you went from 48k to 100k, you would not need to worry about this.
We were pinged by Marketo well before our renewal when our DB size grew rapidly to almost 100% above our limit. I just check the graphic in the Lead Database tab occasionally to stay on top of this.
With one company, our database limit was 250,000, and we grew to 420,000 known records (68% over the limit). We were this way for several months, and there was only some pressure to get it back down.
With another company, our limit was 1,000,000, and there seemed to be almost immediate pressure to get it down when we were at 1,050,000 (only 5% over our limit). Maybe that depends on the account manager, or whether you're close to renewal, or something else. Like Elliott, I just check the graphic often and begin chipping away with deletion campaigns as needed. Generally, Marketo has been patient with our management of the database.