Specialty grain percentage

When putting a recipe together, is there a guide or rule of thumb for the max amount (percentage wise) of specialty grains you should use in a recipe?

Short answer: sort of...

Long Answer: You can look up recommended max amounts of specific grains, but the important things is that you need enough diastatic power to ensure conversion

It depends on the style.
In general, most brewers try to stick to 5 to 10% max for crystal (there are, of course, many exceptions).
A low alcohol dark beer, like an English mild, could use up to a third (or slightly more) specialty grain (crystal + toasted + roasted) to get body and flavour with the low alcohol, but one-third specialty grain in a 9% DIPA would be too much.

Like FAB said, you also have to make sure there is enough diastatic power from the base grain to ensure conversion - that's the only 'real' limiting factor.

I think I understand the diastatic power concept. I'm newer to all grain, and first brew is going to be a SMaSH. Once I do the SMaSH I want to build on that recipe with other specialty grains.

Depending on the style of beer, you may find it unnecessary to use any specialty grains.