Lets see:

Gmail - no

Outlook - no

Protonmail - no

Tutanota - no

GitHub - yes

Dropbox - no

Standard Notes - no

FC2 - no

Amazon - yes

iTunes - no

GOG.com - no

Steam - no

itch.io - no

Twitter - no

YouTube - no

FC2 (repeated above)

HN - yes

Stack Overflow - no

Discord - no

Linkedin - no

Netflix - no

Spotify - no

Mubi - no

Grammarly - no

eBay - yes

Bitwarden - no

SimpleLogin.io - no

Firefox Relay - no

Firefox Lockwise - no

So, 28 listed (removing the duplicate) yet only have four accounts.

> What is your approach?

A local (no cloud, not 'online') password manager. A manager is the only /reasonable/ alternative for tracking all the various sites that all want one to register an account for this or that use.

> Do you maintain 100+ accounts?

My password manager has about 350 "entries" in it -- not all of the "entries" are "online accounts". So taking 75% of that as likely "online accounts" that comes out to about 262 "accounts". Do note that most of those are retailers from which I've purchased once, but that failed to have a "guest purchase" option. The 'core' day-to-day usage entres over any given time period amounts to likely about 5-15 total.

> Don't you think having 100+ accounts (even with a password manager) takes a mental toll on you?

Not at all, because all the effort of "remembering" is handled by the manager. That is one of the benefits of using a password manager, all the effort of remembering "do I have an account here" and "what is the user-id/password for that account" is handled by the manager, which is a massive mental simplification. When a given account is needed, a short search either finds it, or I just add another to track it for reuse later.