100 characters, hidden
One hundred characters, never shown to anyone, indexed for search. It is the least glamorous field on your listing and the easiest one to waste a third of without noticing.
The same hundred characters. Spaces, plurals and words already in the title are the three ways the top bar loses a third of its space.
The short version
"a,b,c" not "a, b, c".
The store builds the phrases for you.
It is counted once, wherever it appears.
All of it is either handled or irrelevant.
Unused characters are the only true waste.
Apple splits the field on commas. A space after each comma is a space spent on nothing, and across twenty terms that is twenty characters, a fifth of the field, gone.
Write it as one unbroken run: "recipe,dinner,mealplan,cooking". It looks wrong and it is correct.
No spaces after commas. That habit alone costs about a fifth of the field.
Apple combines your keywords with each other and with the words in your name and subtitle to match multi-word searches. You do not need to write the phrases out.
Putting "meal planner" in as a phrase spends the characters twice over, because "meal" and "planner" as separate entries already let the store build that phrase and every other combination the words can make.
Anything in your app name or subtitle is already counted. Repeating it here is the single most common waste in the field, and it is invisible unless you check the three fields together.
Also leave out your own brand name, your category name, and the words "app" and "free". Apple already associates you with your category, and nobody searching adds the word app to find one.
Your name, your subtitle, your brand, your category, "app" and "free" all belong out of this field.
Apple handles simple plural forms, so entering both "recipe" and "recipes" spends eight characters to say one thing. Pick the singular and move on.
The same goes for obvious stems. One form is enough for the store to match the others.
What people ask
100 characters, per localization.
Apple gives you 100 characters in the keyword field for each localization you publish. It is never shown to users. Your app name and subtitle are indexed separately and do not count against it, so a single localization gives you 160 indexed characters in total.
No. Separate terms with commas only. A space after each comma is a character spent on nothing, and across a full field that is around a fifth of your space gone for no benefit.
No. Apple handles simple plural forms, so adding both the singular and the plural spends characters to say the same thing twice. Use the singular.
Apple's review guidelines prohibit using trademarked terms you do not own in your metadata, and listings doing it have been rejected. It is also usually ineffective, because searches for a specific app name are dominated by that app.
The rest of it
When you want the numbers
The App Rating checker scores any App Store app out of 100 against the leaders in its own category. Free, no account, no card.
ASO Studio