From my notes - there is a Citrix KB article on this also but I don't have that recorded.
If you create a catalog and the numbering is out of line, eg you create VDAs 01 - 10 and need to delete VDA05, you can recreate in sequence like this:
Get-AcctIdentityPool -IdentityPoolName 'xxxxx 2016 Desktop'
AvailableAccounts : 0
Domain : domain.local
ErrorAccounts : 0
IdentityPoolName : domain 2016 Desktop
IdentityPoolUid : xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
InUseAccounts : 10
Lock : False
MetadataMap : {}
NamingScheme : CTX-W2016APP-##
NamingSchemeType : Numeric
OU : OU=xxxxx,DC=domain,DC=local
ResourceLocationId : xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
StartCount : 11
TaintedAccounts : 0
WorkgroupMachine : False
ZoneUid : xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
Scopes :
TenantId :
Note the StartCount.
Set-AcctIdentityPool -IdentityPoolName 'xxxxx 2016 Desktop' -StartCount 5
Create 1 new VDA as normal, then reset the count back to where it needs to be
Set-AcctIdentityPool -IdentityPoolName 'xxxxx 2016 Desktop' -StartCount 11