Powershell Make a function for directories and add a prefix at the end - function

Question

Make a function that makes 3 directories with the name John_S and add the prefix and appended with the number 1, 2, m
Example
1. John_S1
2. John_S2
3. John_S3
Use a loop (ForEach)
Use a variable for the number of iterations
What I have so far...
$DirName = "John_S"
function mulcheck {New-item "$DirName"}
$i = 1
foreach($DirName in $DirNames)
{$newname = $DirName Rename-Item $($DirName) $newname $i++}

Answer 1

The easiest way to generate the numbers 1 through 3 is with the .. range operator:
foreach($suffix in 1..3){
mkdir "John_S${suffix}"
}
To make the function re-usable with something other than John_S, declare a [string] parameter for the prefix:
function New-Directories([string]$Prefix) {
foreach($suffix in 1..3){
mkdir "${Prefix}${suffix}"
}
}

Answer 2

If I understand your latest comment correctly, you want a function that takes the name of a new folder and checks if a folder with that name altready exists in the rootpath. When that is the case, it should create a new folder with the given name, but with an index number appended to it, so it has a unique name.
For that you can use something like this:
function New-Folder {
[CmdletBinding()]
param (
[Parameter(Mandatory = $false)]
[string]$RootPath = $pwd, # use the current working directory as default
[Parameter(Mandatory = $true)]
[string]$FolderName
)
# get an array of all directory names (name only) of the folders with a similar name already present
$folders = #((Get-ChildItem -Path $RootPath -Filter "$FolderName*" -Directory).Name)
$NewName = $FolderName
if ($folders.Count) {
$count = 1
while ($folders -contains $NewName) {
# append a number to the FolderName
$NewName = "{0}{1}" -f $FolderName, $count++
}
}
# we now have a unique foldername, so create the new folder
$null = New-Item -Path (Join-Path -Path $RootPath -ChildPath $NewName) -ItemType Directory
}
New-Folder -FolderName "John_S"
If you run this several times, you will have created several folders like