Conditional Formatting value in a matrix against a different source's column

Create a measure to count the inputs (e.g., CountOfInputs = COUNT(InputsTable[InputID])).

Create a measure to compare the count against the threshold value from the other table:

If the threshold table has one value:
MatchThreshold = IF([CountOfInputs] = SELECTEDVALUE(ThresholdTable[TargetCount]), 1, 0)

If related by category or another key:
MatchThreshold = IF([CountOfInputs] = RELATED(ThresholdTable[TargetCount]), 1, 0)

Optionally, create a color-returning measure:
MatchColor = IF([CountOfInputs] = RELATED(ThresholdTable[TargetCount]), "#FF0000", "#FFFFFF")

Go to the matrix visual in Power BI.

Click the dropdown arrow on the values field.

Choose "Conditional formatting" > "Background color" or "Font color".

Set "Format by" to "Field value".

Choose the color measure (MatchColor) or logical measure (MatchThreshold) as the formatting field.

Apply the formatting.