I am going to purchase an account for Minecraft Java Edition, however, I wanted to know if I could use the same account on multiple computers at the same time, so I could be playing Minecraft on my laptop and my little brother could be playing Minecraft on another laptop at the same time as me on the same account (this is for singleplayer not multiplayer).
Edit: This answer is incorrect. See fasterthanlight's answer.
You can use a single account on multiple computers but not at the same time. Your brother can use it only while you are not playing.
Since The Microsoft Acquisition of Mojang/Minecraft
It can permanently cost you the right to play the game. It's not worth it to save $26.95.
The following products, apps and services are covered by the Microsoft Services Agreement, Minecraft games Minecraft Realms Plus and Minecraft Realms
BY DOWNLOADING OR USING THE APPLICATION, OR ATTEMPTING TO DO ANY OF THESE, YOU ACCEPT THESE TERMS. IF YOU DO NOT ACCEPT THEM, YOU HAVE NO RIGHT TO AND MUST NOT DOWNLOAD OR USE THE APPLICATION.
If you comply with these Terms, Microsoft grants you the RIGHT TO install and USE ONE COPY of the software per device on a WORLDWIDE basis for use BY ONLY ONE PERSON AT A TIME.
The software is licensed, not sold, and Microsoft reserves all rights to the software This license does not give you any right to, and you may not:
Separate components of the software or Services for use on different devices SINGLE-PLAYER AND MULTI-PLAYER
You may not: lend the software or the Services, unless Microsoft expressly authorizes you to do so;
You may not: transfer the software, any software licenses, or any rights to access or use the Services;
You may lose access to or capabilities of Digital Goods, or have the nature of your access changed if you violate the Terms or these Rules.
Since The Microsoft Acquisition of Mojang/Minecraft there is no difference between singleplayer and multiplayer on the Microsoft end because you still have to log in to your Microsoft account to load the game and all worlds are technically a server even if it is hosted internally.....