Depends on boat, weight and waterbody. There is no magic number. People run from as little as 9 to as high as nearly 12 (with wake boats). One of the typical misconceptions is people try to gauge their wake by looks. A great looking wake, might not ride worth a damn. If I was to speculate speed isn't your issue, you need add more weight and basically gouge/displace a larger wedge of water out.
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