D5D: Drive Sizes and Partitioning

I have 5 4TB drives and one is flashing red. How many 6TB drives can I add before I need to create a new paritition?
I would like to add a few 6TB drives.

On another drobo I have 5 6TB drives. How many 10TB can I add? I lalready have that one broken into two partitions.

Not sure exactly what model you have, but max drive & volume sizes are listed here:
Maximum Volume Size and Largest Size Drive (drobo.com)
Please pay extra attention to what drives you purchase SMR technology is getting more common & isn’t supported, see here
Can I really use any hard drive I want in Drobo?
In the case you’re using 16TB volumes & wish to use 64 (if supported) see here:
Drobo 5N; Drobo 5D: 64TB Volume Support

As for when you’ll exceed a given size I’d suggest using the capacity calculator:
Capacity Calculator - Drobo