You could if you wanted to, but vCenter will normally throttle itself.  So you could have all 50 queued up and only a subset of those would actually be active jobs.

One method would be to introduce an arbitrary count and sleep method.  Or another more accurate, but would require a bit more coding would be to look @ the tasks and only start another job if the active task count is less than 4 or 5.

Here is the first method as an example:

$filepath = "D:\PathtoCSV\filename.csv

$csvobj = import-csv $filepath

$Count = 4

foreach ($row in $csvobj) {

     $Count --

     $vmobj = get-vm $row.vmname

     $ds = get-datastore $row.destds

     $vmobj | move-vm -datastore $ds -runasync -confirm:$false

     If ($Count -lt 0){Sleep 1500; $Count = 4}

     }