When your ABM program is small, the margin for error is really tight. Go after the wrong accounts and your ROI is going to be poor, making it impossible to attract further investment. This is why you have to focus your efforts on the most viable accounts while still making some effort towards the others.
You can tier your target accounts so they get the right level of attention and resources. Tier one would get the most dedicated marketing and sales attention to get them to close while tier two would likely contain a larger list but will receive less investment.
To help tier your accounts, you'll need a scoring system that can be made up of firmographics, cumulative behaviors, and intent data. Then you can assign specific activities to each tier. Here's an example of this from Marketo: