And it should be noted that you almost certainly don't have any guarantee - either verbally or in writing.But I don't know of any ISP that can guarantee that every customer can simultaneously download at their subscribed service level
Typical Internet access connections use "statistical multiplexing". That is, they deliberately oversubscribe the lines on the (usually correct) assumption that you won't find everybody trying to use the line at the same time. Residential connections may be oversubscribed at a ratio of 10:1 or even higher. "Business class" Internet connections are typically oversubscribed at around 4:1, which is why those lines usually cost more. If you read your contract, you will see that the ISP explicitly disclaims any guarantee of any amount of service.
If you want an actual guarantee, including contractual penalties for failing to deliver, you can get it, but it's not cheap. For this, you need to get a "leased line" from your service provider. There will be no oversubscription and you will have an SLA (service-level agreement) that promises some minimum level of service and has legally-actionable penalties if they don't meet that agreement. And you will pay a small fortune every month for such a line.
Leased lines are usually used as point-to-point links that interconnect data centers or large corporate campuses. There is almost no good reason why a small business or an individual needs that level of service.