Having the option to add a prefix to each individual client would be a huge help to me.

I strongly agree with Ryan's suggestion, and ideally I would like full control of the way my invoice numbers are generated, but I'll take the customer specific prefix as a big step in the right direction.

My ideal system, and the way I currently generate manual invoice numbers is as follows:

CST-170501

Where:
CST = Client specific prefix
17 = The current year
05 = The current month
01 = Incremental number. In this case the first invoice for that client in that month.

With this system, I can tell, at a glance, who the customer is and how old (or overdue!) the invoice is.

E.g. with the following number, MWG-150305, I can tell I invoiced Midwest Grill in March two years ago and did at least five jobs for them that month.

This system is perfect for me as I am unlikely to invoice an individual customer more than 99 times in a single month. If other businesses are sending up to 999 invoices to each client, every month the incremental number could easily be increased to three digits.

when using the invoice self generating number setting: this would be a great tool if one needs to apply different prefixes for different customers.
Example: we sell to store AA and BB so the invoice generator should allow for customization per client so that store AA always gets invoices numbered AA-xxxx and store BB always gets an invoice numbered BB-xxxxx

I just join zoho invoice and I think these option would be great because I want also more customisation for the invoice number.

This is almost a requirement for me, otherwise it's difficult to manually keep track of this and set them manually. I'd like each client to have a field (could be an arbitrary label or custom token).

In the invoice auto-generator, add the option to include these tokens (similar to how address placeholders and other tokens work elsewhere on the site).

Ideally we could have an auto-increment for each client as well. My ideal setup would be something like this:

CLA-16-001

Where:
CLA = Custom Token for Client (Client A for example)
16 = Placeholder for current year
001 = Auto-increment ID, **specific to client**.

It's important my clients have no insight into the total number of invoices I generate or have generated between 2 invoices to them!

The solution above would be ideal, but any step in this direction would be appreciated.

there is currently no option to set prefix letters for a particular customer with the number of the invoice changing as they are created as recurring invoices. I have 37 customers who I would like to have a different prefix for.

definetely - this is a must have option if you have more branches, offices, different kind of clients or / and services and you wonna have nite bookeeping !