Number series

In many projects I have learned: New company = new number ranges / number series / new methodology of number assignment. Here is my approach to how I would assign customer numbers, supplier numbers and document numbers via the number series.

In Navision or Business Central, you have many variation options via the number series, number series lines and number series links to assign number series according to all the rules of the art. Numbering in steps of 2 or 10, prefixed fixed parts such as years or letters, e.g. RG or GS for invoices and credit notes, several blocks, e.g. for subdividing business areas or departments: All this is possible with the number series from Navision or Business Central!
Nevertheless, I would not use this flexibility. Here you will find my recommendations.

Document numbers

100000 : Offers (VK)
200000 : Orders (VK)
300000 : Delivery notes (VK)
400000 : Posted invoices & credit notes. Both document types have a number with RG400000 when they are created and only receive their final number when they are posted. This has the advantage that when you delete an unposted invoice, Navision does not ask you "Deleting the document has resulted in a gap in the number series. Would you like to create an empty record?
500000 : Sales reminder (unposted:VKM500000, for the same reason as for the posted sales documents)
600000 : Inquiries (EK)
700000 : Orders (EK)
800000 : Deliveries (EK, no other meaning)
900000 : Invoices & credit notes (as in the UK, no other meaning)
The number of jobs should be based on the number of employees, preferably with no increase in jobs for the foreseeable future (at least the next 10, better 20 years)!
So: If you write 8,600 offers per year, you should expect at least 50% growth, better 100% growth, i.e. 16,000 offers per year. Over 10 years, that's 160,000 offers. In order to avoid a jump from 6 to 7 in the 8th year, you should number in seven digits, i.e. start with 1000000 (1,000,000).
The record type with the most records per year determines the number of digits for all records. In most cases, 6 digits (which is only 5 digits due to the leading code!!!) will suffice. 6 digits also has the advantage that the document numbers are differentiated from the customer and vendor numbers.

Alternative: Alphanumeric numbering

e.g.: AB100000, RG100000:
I do not recommend this, as you and your customer/supplier can enter a number less fluently this way. Test: Type in 100000, preferably on the right-hand side of your keyboard. And then type in RG100000: you have to use 2 parts and probably 2 hands.

Alternative: Year numbering

e.g. 19.10000, 20.100000, 21.100000
I also clearly advise against this. Why?
1-You have to remember to change your number series every year. Navision makes this quite easy for you with year-dependent number series lines: you can pre-maintain the number series in Navision and Business Central years in advance.
2-You have more work if, for example, you want to create another receipt in January for the previous December. A program adjustment could make things easier here, but why fix what you can avoid?
3-Every year, your customers and suppliers gain insight into the capacity utilization of your business operations. If you create an invoice with 20.2000007 on March 5th, then your customer (supplier) knows exactly that you have only written 7 invoices (orders) in this quarter. And this disadvantage is inherent in annual numbering if you start each year cleanly.

Vendor and customer numbers

Here I would roughly stick to the Datev, i.e. 5 digits.
This is a common standard.
Datev also used to sort by initial letters, e.g. 10000 for A..., 11000 for B...., 12000 for C..., but this is no longer common. Especially in Navision & Business Central, search terms (matchcodes), any sorting by location or postal code, sophisticated search routines ensure that you can find a customer (debtors) or supplier (creditors) wonderfully even without this logic and get nicely sorted lists from Navision / Business Central in any reports, regardless of the number / number ranges.

Item numbers

Oops... different topic. Not because of Business Central and Navision, but because of catalogs. Webshops, price lists, general information that goes to the customer. Here we should take a closer look at your articles and consider whether a speaking, logically grouped article number is too much of a good thing or just the right thing. Here, it may also be a good idea to number article numbers in Business Central & Navision with letters in the number ranges / number series.