How can I say "usually one, sometimes multiple" at the end of a sentence?

I would just use an extra sentence.

A logic gate performs a binary operation and produces a binary output. Most gates have a single output, but some have multiple outputs.

You can say it like this:

usually produces one, but sometimes multiple, binary outputs.

I would change the sentence entirely.

[...] and produces a certain, predefined, number of binary outputs.

or

[...] and produces at least one binary output.

. . . and produces usually one, sometimes multiple, binary outputs.

The problem with this is that you're using parenthetical information—and you're allowing it to change the grammar of the surrounding text, something that you should not do.

The phrase sometimes multiple is nonrestrictive.

The essential part of the sentence is this:

✘ . . . and produces usually one binary outputs.

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In order for it to be strictly grammatical, the final word needs to be in the singular:

✘ . . . and produces usually one, sometimes multiple, binary output.

However, despite it's technical correctness, it sounds so bad that it shouldn't be used either.

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You could rewrite it in a fashion similar to this:

✔ . . . and usually produces a single binary output—but sometimes more.

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However, I think the most effective way is to rephrase it so the final part of the sentence takes a parallel form with what comes before:

✔ In electronics, a logic gate is an idealized or physical device implementing a Boolean function; that is, it performs a logical operation on one or more binary inputs and produces usually one but sometimes multiple binary outputs.

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To explain, this is allowed:

one or more binary inputs

Even though the phrase starts with one, inputs is closest to a plural noun—so it takes a plural form.

Similarly, we have this:

usually one but sometimes multiple binary outputs

Also, we can make this grammatical change because there is no parenthetical, nonrestrictive information that should not be affecting the surrounding grammar.

Since a Boolean function produces one binary output by definition, it's not really correct to say what you're trying to say.

How about something like this:

In electronics, a logic gate is usually an idealized or physical device implementing a Boolean function; that is, it performs a logical operation on one or more binary inputs and produces a binary output. Sometimes devices that produce multiple binary outputs are also referred to as gates if _______.

I leave the blank blank, because I'm not actually familiar with anything that has multiple outputs that I would refer to as a "gate". Latches and flip-flops, for example, would be circuits composed of multiple gates.