A ThingSpeak channel can be updated at most once a second with a paid license, or once every 15 seconds with a free license. If you have multiple devices updating the same channel, and their clocks are not exactly synchronized and updates staggerred to prevent one device trying to update a channel before the appropriate time has elapsed since the last update by another device, you will run into collisions like you are reporting. Even if your devices could synchronize their clock, it is likely that you cannot control exactly the time it takes for the HTTP request to make it to ThingSpeak servers, and it is quite possible that you will run into the issue of colliding updates.
My recommendation is to set up a single channel per device and use the channel tags linked to a serial number of the device to disambiguate. That way, when a device powers up, it uses the channels API to determine if a channel exists for it (search by tag) and if not, the device dynamically creates the channel. On subsequent disconnects and reconnects, it finds and updates the channel it had set up on initial connect. This is what I do with my dozen or so ESP32 devices.
I know for a fact that this approach scales to many 1000's of devices.
