I plan to take ferries to the Outer Hebrides, Orkneys, Skye, Mull/Iona in July 2018 (busy season I know.) I will be taking my rental car. Has anyone had experience making these voyages with a car? Are the trips/day and number of cars that can be accommodated/trip limited? How far in advance do I need to make reservations with a car?
Did a trip around the Highlands (Glasgow, Oban, Mull/iona, Skye, Orkney and back to Edinburgh). Yes, the ferries have a daily schedule and since it's a ferry, the number of cars allowed is limited as there is only so much space. Since you will be going during the busy season, I'd recommend you make reservations now, especially if you have very little flexibility in your travel plans.
This company operate ferries on the west coast of Scotland:>https://www.calmac.co.uk
You will get a better response by posting this in the Scotalnd forum - which locals are more likely to monitor.
I can only speak to the ferry to and from Skye. I took it last August and made my reservations just a couple of days in advance, once my plans were firm. It was necessary though, all of the ferries on the afternoon I went were sold out when I arrived around noon. I would think it would be similar for the other ferries too.