To generate a generic recurring billing item for a customer (client) in WHMCS, that will be billed multiple times on multiple invoices you follow this procedure:
In the client profile page, you go to the "Billable Items" tab.
Click on the "+ Add billable Item" link (on the right) and put a generic description, amount etc.
In the "invoice action "select "Recur every <N> <days/weeks/months etc> for <Y> times". Leave "invoice count" to 0 (this is the counter for WHMCS to keep track of the number of invoices generated already.
Save changes.
WHMCS will generate multiple invoices for this billing item, up to the selected number of times. This happens on the cron run, once per day at the predefined schedule. It might be possible to select the item and click to "invoice now". It works for single-shot billable items. I have not tested if it works on items invoiced multiple times.
As invoices are generated, this billing item will move to the "Invoiced Items" list on the same page. This is a table that contains links to the invoice numbers. A multiple invoiced item will have multiple invoice links to each invoice.
Background: I use this whmcs feature to invoice customer-specific jobs without the need to create a package, create an order etc. Mostly useful for ad-hoc consultancy work where I bill the customer a fixed amount per month on a regular basis, for work that does not apply to other customers. Or, when I need the customer to pay a bigger amount in multiple installments (i.e. say 1000 USD, 250 USD/month in 4 months). This way they get a single invoice, reminders, overdue notices on each installment.
Le me know if you need more info.
Thank you.