Than you

yours in peace
nhriley

To put it most simply, your 12 Volt transformer puts out a certain maximum current, which will be marked on it’s specification label, such as 5 Amps, or 5A.
Reckon that each HO locomotive takes a maximum of 1/2 Amp, so a 5 Amp transformer would give you a maximum of 10 locos running at the same time. But if you have accessories such as lighting on the layout, from the same transformer, add it’s current to that taken by the locomotives.
The switches only take current for a fraction of a second when changing, so they can be ignored.
If the total current taken by everything is equal to, or less than, the tranformer’s rated output current, you’re OK.
(If your 12 Volt transformer is only marked with a power rating, such as 40 Watts (40W), reckon 6 Watts per locomotive, and add up the power ratings of the other items in the same way.)

If model railways is your chosen hobby, you would be wise to get a basic course in electricity, such as starting off with the electricity section of a school physics text book. Also, most beginner’s books on model railways will provide the basic electrical knowledge.