Work out how much grass seed you need...
The “standard” mix; landscaping and lawn turf
They come in lots of different shapes and sizes, but the overwhelming majority of crops fit into the large bracket of “general landscaping/lawn turf”.
Exact seed mixtures vary from grower to grower; here’s a template as useful starting point:
20-30% perennial ryegrass
20-50% strong creeping red fescue
20-50% slender creeping red fescue
0-20% Chewings red fescue
0-10% smooth-stalked meadowgrass