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so if i have a unit of 5 plague censurs vs a unit of anything else of 5 across

for the toughness test, is it 1 touness test per a model or is it a toughness test for each censur next to that model?

so in the example is it 13 toughness tests or 5 toughness tests

i believe its 5

One toughness test per model in base with one or more censer bearers, so 5.

It lays this out very explicitly in the Skaven codex.