Saving changes if adding to context - entity-framework-core

Question

EF Core 2.2.x
The question Im trying to answer is if I can limit my call of SaveChanges to only once or do I need it each time I add an entity, since in the next lower level I need to query the DbContext to see if the item exists.
Am trying to "merge" some data between two sets of similar tables. So in the process of adding records and drilling down the hierarchy, I have to first see if a record exists and if not, then add it.
/*
List<StagedOrder> sorders;
StagedOrder
Id
Number
InsertDate
Notes
StagedOrderLine
Id
ItemNo
Qty
StagedOrderKey
*/
foreach( StagedOrder s in sorders) {
// get order
Order o = (from a in ctx.Orders where a.number = s.Number select a).FirstOrDefault();
if(o == null) {
o = new Order() {
Number = s.Number,
InsertDate = DateTime.Now,
Notes = "imported order"
};
ctx.Orders.Add(o);
// ctx.SaveChanges() ???
}
// get lines for orders
OrderLines ol = (from a in ctx.DetailLines where a.OrderKey = o.Id select a).ToList();
if(ol == null) {
foreach(StagedOrderLine l in s.StagedOrderLines) {
ol = new DetailLine() {ItemNo = l.ItemNo, Qty = l.Qty, OrderKey = l.Id}
ctx.DetailLines.Add(ol);
ctx.SaveChanges();
}
}
}
// can I limit my call to savechanges only once here?
// ctx.SaveChanges();
Also, should I create a new DetailLine using an Order.Id or an entity reference, eg.
ol = new DetailLine() {ItemNo = l.ItemNo, Qty = l.Qty, Order = o}