All engagement via Re-Ex is genuine, therefore it is difficult to predict exactly how much engagement a campaign will receive. There are various factors that can affect the success of a campaign including time of day and how many similar campaigns are currently running, among others.
Luckily, there are a few things you can do to increase your campaigns visibility:
Make sure you are using a genre from SoundCloud's 'main' genre list.
Add relevant tags along with the 'main' genre to further specify your target audience.
Turn on auto boost to make sure you use your free boosts effectively (pro plan only).
Set custom campaign rules to refine your campaign's target audience (pro plan only).
Use our Campaign Accelerator feature to increase activity on your campaign (pro plan only).
Set your campaign as a Featured Campaign to increase visibility (pro plan only).
For more info on improving your campaigns success check out this article!