Crusader Kings III

Start a new game in AD 867 as Jarl Dyre 'the Stranger' of Könugarðr (make sure you have enabled Ironman mode!). Könugarðr is located next to the huge area of Khazaria.

After the selecting the Jarl, you will want to select the Intrigue lifestyle along with Temptation Focus (increases fertility). You will be able to select the second tree which includes "Like Weeds in a Garden" which increases fertility also.

After that, select as many concubines as possible to max out children being produced. It will also help if you divorce your current wife ASAP as their fertility rate is decreased due to their sexual orientation. You only need 100 piety to divorce and you start off with 50/+2.0.

As the Jarl is only 22, you have plenty of time to produce lots of children!

To make this easier, you must start with a young character and ideally, marry a young spouse with the Fecund Trait. Alternatively, you can start as Sultan of Seljuk in 1066. He starts with 9 children and you can use the Seduce scheme on the wife to obtain the last one.