As you have 5 pumps, I hope you will be creating 5 separate equipments & one Maintenance Plan with 5 Maintenance Items.
First 3 pumps (from Supplier A) will have 4 internal activities & 2 external activities. Second set of 2 pumps (from supplier B) will have same 4 Internal activities & only one External activity.
Are all the pumps are of same type in business view?? What is the reason that Supplier A gives 2 operation & Supplier B gives only one operation as External Maintenance??
You can create 2 task lists viz., one for 1st group & another one for 2nd group. Maintenance Item will contain one pump from either 1st group or 2nd group. So you can assign appropriate task lists to that Maintenance item.
I am not sure about your actual query. Please read my understanding mentioned above & rephrase your query.