I only have about 3, so I’ll definitely need more. I was just finding that my snails weren’t lasting long, after about a month they would die. My most recent ones have been doing well though.I feel that reefcleaners exaggerates a bit how many snails etc you need, so I wouldn't follow that list exactly. But yeah, definitely add more!
For reference, I have about 12 algae eating snails in my 50 gallon currently; still planning to add more (was waiting for a toxic dino outbreak to finish).
If you have an LFS so it's convenient, I like the advice to add just a few each week/2 weeks, so you can see how the algae level responds and not risk overstocking on snails.