Every single one. Whether smurfing or not. Whether intending to, or not. Repeat counts disrupt the fidelity of your SR curve, which is supposed to track players by % skill distribution.
To be even more specific: every single alt account adds disruption on the non-robust metrics. 1 of them, 10 of them, N=10,000 of them. The more you inject, the worse it gets. Always. This is bad for your ladder and shams away your esport (but not your $$$$).
Every single alt will monotonically deteriorate rank/ladder integrity - i.e. what SR actually measures, how it scales, and how correct it is. Whether it tries to or not. You can’t have people taking multiple IQ tests without replacement. It throws off the sampling, and makes ranking incorrect. The overall shape of the curve doesn’t necessarily have to ‘change’ to augment disparities in the indexing structure.
There is no need for data, personal experience, or annecdotes here. We can settle it with math. Show me with math how alts don’t ruin the meaning of SR and I’ll show you bad math