I think it depens, it's not lineal, each workflow has a different weight in the total load it creates depending on:
But, for a rule-of-thumb you can use atlassian sizing guide:
From my point of view, having a large number of workflows creates problems with the administration first.
A workflow only really imposes load when it is being used, not just because it's in the system. There are far more important things, like complex and multiple permission schemes, active users, number of issues, complexity of fields on issues and so-on.
I completely agree with Gaston - your admins will be overwhelmed by too many workflows to look after before you'll be able to say "too many are causing a load"
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