at least you must have 25% of overlaping , with this the user have enought time in case in a regular walking for allow the normal negotation process between both ap´s for a transparent roaming for the user. by rule with 10 a 15 % its enought but i recommend 25% in my experience its better if you have the capacity for that deployment.
here is a reference for the overlap
https://support.huawei.com/enterprise/en/doc/EDOC1100193623/b87472db/wireless-roaming-design