How can I force Powershell to return an array when a call only returns one object?

Define the variable as an array in one of two ways...

Wrap your piped commands in parentheses with an @ at the beginning:

$serverIps = @(gwmi Win32_NetworkAdapterConfiguration 
    | Where { $_.IPAddress } 
    | Select -Expand IPAddress 
    | Where { $_ -like '*.*.*.*' } 
    | Sort)

Specify the data type of the variable as an array:

[array]$serverIps = gwmi Win32_NetworkAdapterConfiguration 
    | Where { $_.IPAddress } 
    | Select -Expand IPAddress 
    | Where { $_ -like '*.*.*.*' } 
    | Sort

Or, check the data type of the variable...

IF ($ServerIps -isnot [array])
{ <error message> }
ELSE
{ <proceed> }

Force the result to an Array so you could have a Count property. Single objects (scalar) do not have a Count property. Strings have a length property so you might get false results, use the Count property:

if (@($serverIps).Count -le 1)...

By the way, instead of using a wildcard that can also match strings, use the -as operator:

[array]$serverIps = gwmi Win32_NetworkAdapterConfiguration -filter "IPEnabled=TRUE" | Select-Object -ExpandProperty IPAddress | Where-Object {($_ -as [ipaddress]).AddressFamily -eq 'InterNetwork'}

If you declare the variable as an array ahead of time, you can add elements to it - even if it is just one...

This should work...

$serverIps = @()

gwmi Win32_NetworkAdapterConfiguration 
    | Where { $_.IPAddress } 
    | Select -Expand IPAddress 
    | Where { $_ -like '*.*.*.*' } 
    | Sort | ForEach-Object{$serverIps += $_}