What are the limitations on the number of simultaneous requests from Windows 10?

htb's response is written from the perspective of Windows port depletion, but as WebSurfer commented, it is highly likely that Windows has not run out of ports.

Apart from that, IPoE (IPv4 over IPv6) is popular in Japan's Internet environment these days.In the case of IPoE, it is the ISP side that manages the actual IPv4 port, and the ISP side may limit the number of concurrent connections to prevent port depletion.Restrictions vary from ISP to ISP, but clients may experience slow response and strangely wait times.In that case, the client may consider the timeout.

You can determine whether IPoE limits the number of concurrent connections by switching the connection method to PPPoE.

(FTP is a 1 file 1 connection, so ftp client development is easily depleted...)

Anyway, this question is a XY problem, and you should ask for specific issues (request failures) instead of guessing and asking (window 10 concurrent requests limit).

We cannot identify it from the body of your question, but port depletion is a common issue, so please refer to this troubleshooting.
Troubleshooting Port Exhaustion Issues

Check the netstat event log when the problem is reproducing.