IPv6 Subnet Calculator
Split the prefix from your ISP into subnets and see which bits belong to the ISP, which number the subnet and which are left for hosts
Parameters
CIDR notation, e.g. 2001:db8:c040::/44
/64
For EUI-64 conversion, e.g. 00:00:5e:00:53:01 (RFC 7042)
of
ISP prefix
Subnet
Interface ID
Results
RouterOS
2001:db8::/32 is the documentation range (RFC 3849). /64 is the usual LAN subnet length for SLAAC.