How many horses can you park at your home? I have been thinking about this and this is what I have come up with so far:

1) Shadowmere will stay where you leave him if you mount another horse.
2) I think Frost behaves the same way.

That gives you two horses that will stay at your home as long as you mount another horse before fast travelling.

What about a horse that you own other than the previous two? If I buy a horse in Riften and bring him to Lakeview then mount another horse (arvaak) will that horse stay there or will he start heading back to Riften?

If you have your steward buy a horse, and DON'T ride him, he will stay, otherwise the stupid thing will be with you as soon as you fast travel or go anywhere by carriage, and do that always until he's dead.

If you do ride him, I would bet you can still get him to stay at the manor by simply mounting another horse. Summon arvaak, get on him, then get off, and your manor horse should stay put.

Not sure about this, but I will give it a try.

Added: Thanks for reminding me about the horse your steward can buy. That gives me at least three horses.

I didn't try it with Arvak, I hope it works!

Alright, I just checked out the manor horse. Here's what I found.

1) I bought the horse
2) I got on it and rode it for 10 feet.
3) got off and fast traveled.
4) the horse came with me
5) I rode the manor horse back to Lakeview and got off
6) I summoned Arvak, got on him and rode for 5 feet
7) As soon as I got on Aarvak the manor horse walked back into its stall
8) I fast traveled again
9) no horse
10) walked back home
11) the manor horse was in the stable

I think this means you can have 3 horses in total at your house: Shadowmere; Frost; and the manor Horse. A horse you purchase at one of the stables will most likely start walking back to the those stables the instant you mount another horse.

Excellent!! I probably will never have Shadowmere, but may do the quest to get Frost. I have a character who has Arvak. Thanks for the report!

May not be the cavalry for me, but still information that helps me!

I currently have a normal horse I got from a fort during civil war quest, stable horse and a horse from dawnguard fort. Not quite sure why, I jumped off civil war horse to run inside fort and contiune quest. I left the fort jumped on what I thought was civil war horse but was dawnguard horse (they look identical) and fast travelled home. On load I turn round to see twins and my stable horse. Going to see if I can get the other horses

MMMMMMMMMmmmmmmmmmmmm horses, dragons lurrrrrrve horseymeat, be prepared for many dragon attacks.

Any idea what happens to Frost if you fast travel with him to a stable and buy another horse?

I'm looking for ways to retire Frost since he dies EVERY SINGLE time bandits attack my house. I'd prefer to let him live somewhere quiet, but maybe it's just his time to go.

As long as you mount and ride another horse, Frost will stay where you left him. If you are worried about his safety, I would leave him at the Whiterun Stables, they seem the safest. I get dragon spawns all the time at the Windhelm, Riften, and Solitude stables.