I don't think there is an actual number (know this is not the most helpful response) due to the number of variables that could happen.
For instance, doing 5 over 5 years is very different to doing 5 over one cycle. Its also very different if they are also all very recent rather than some many years ago.
One person doing 5 schemes could have had 5 very different reasons for being unsuccessful at those firms, while the next could be getting consistent feedback that they need to develop in a certain area.
And then someone could have done 5 vacation schemes at 5 very different firms, while another may have applied to all 5 MC firms.
Generally though if you have done multiple vacation schemes (3+) in one cycle and are rushing into applying for the next cycle to either the same or similar firms, and got similar feedback, I'd say it is more of a concern. But if the variables are more varied (like those above), it is less of a concern, especially if you can show development/growth or can identify why you have finally found the right firm for you.