How many dependents can I have on an educational visa in Thailand?

It's exactly as

@Stuart ********

's stated. There is no official limit on the number of "piggy back" (dependent) visa/extensions that can "cling-on" to yours. Your wife and children can indeed get Non-O visas and then yearly extensions based on yours.

Now with that being said you need to talk to the university BEFORE you wing your way here because you're better off with everyone getting the correct visa before you show up here rather than comin' in on tourist visas and trying to convert the inside the country. You would get a 90 day Non-ED visa, your wife & children would get a 90 day Non-O visas.

5 Kids. You'd better do more homework. Cheers mate

The children have to be under 20.

The other way around you’re limited to one parent per child. For example if the child is at an international school you can apply for a “guardian” non o based on them attending the school but that’s limited to one parent.

There is no limit to “guardian” visas. If you have three kids each can have their own “following” visas. Your wife too.

Bring your nan also .....

I don't think that's possible, you, your wife and kids over 12 will need a visa?