MT7621A usb device count limit

I found that I cannot connect more than 4 devices to my router.
then the kernel speaks

[    6.278743] usb 1-1.5: new high-speed USB device number 8 using xhci-mtk
[    6.420830] usb 1-1.5: Not enough host controller resources for new device state.
[    6.435818] usb 1-1.5: can't set config #1, error -12

Is this a hardware or software limitation? Can it be changed?

Hi, why you do it "more than 4", only 1 is enough for me.

Which router and which OpenWrt release are we talking about?

I tested it on these two: TP-Link Archer C6U & Hilink HLK-7621 evb.
I used a last snapshot form git