According to Microsoft's SSRS help search:
=Parameters!<ParameterName>.Count
Returns the integer value 1. For a single-value parameter, the count is always 1.
I verified this does indeed work, check the integer returned for the built-in parameter count field.
Allow multiple values on a parameter selection. Checking the value of the above field will let you know how many values the user actually chose.
In my situation, I allow multiple values on company number. This gives users the ability to choose one company to report on or several at once. Per client request, if they choose more than one, display data horizontally. If only one company is chosen in the parameter list, show the data vertically and hide the other tablix.
So my visibility show or hide expression looks like this in the one tablix:
=IIF(Parameters!company_number.Count > 1, True, False)
and like this in the other:
=IIF(Parameters!company_number.Count = 1,True,False)