Dear Edyta,
To achieve the requirement, you need to access the data of the form submission and then let Forms auto-check the number of the bus booking and whether the max number has been reached.
I'm afraid that Forms has no such features and less customization. You may need to use PowerApps to create a form with a SharePoint list as the data source instead of Microsoft Forms to move forward.
Here is a demo for your reference.
As an example, when the submission reaches four, a message pops up in the PowerApps form to notify the users that max number is reached.
The SharePoint list name is Bus Book Request. You can connect it to PowerApps.
Add a text control "The max request number is reached" and set If(CountRows('Bus Book Request')=4,true,false) on Hidden.
After that, when 4 items are added to the list, the message will pop up in PowerApps. In your environment, you can set the formula like If(CountRows('Bus Book Request')=101,true,false)

Your understanding is highly appreciated.
Sincerely
Cliff | Microsoft Community Moderator