Increasing the number of placeholders will not make a difference. At a time, Ezoic will only display a certain number of ads to comply with Google Adsense policies.
For example, they won't show 10 ads for 200 words of content regardless of how many placeholders you set up. The placeholders just help them in testing different spots to optimize your revenue.
How long is your content? If its short content, the payouts are okay. Higher EPMVS are for longer content where they can show multiple ads.
Also, how long do people spend on your page? If people are just spending a few seconds on your page, it isn't possible to show them a lot of ads. So your revenue gets affected.
Also, what's your total traffic? If its low, then they don't have enough data to optimize ads for your sites and again the revenue will be affected.
If you have long form content with a lot of traffic and good time on page, I'd recommend reaching out to their support and asking them why your payout is low. There may be other reasons for your EPMV that I haven't thought of.
To learn how to set up placeholders, I recommend taking a look at this video: