I can't find anything that says about this. I know that on some simulators there are a limit of 32 qubits (quasi simulator) or more. But on real hardware ?
There are several 5-qubit (Belem, Lima, Quito, Manila) and 7-qubit (Oslo and Nairobi) QPUs. These backends are available for free to anybody provided that they are used for educational and research purposes.
On paid basis (you pay for consumed computational time 1.6 USD per second), you have access to up to 27-qubit processors (also 7-qubits and 16-qubits processor are available).
The maximal number of qubits is currently 127 (Washington processor), however, this one and Ithaca processors (65 qubits) are available only to IBM partners.
See complete list of IBM currently available quantum processors here.
Just note concerning the simulator, you can also use matrix-based simulator with up to 100 qubits and stabilizer simulator with up to 5,000 qubits. However, note that the latter one is not universal as it can simulate only circuits composed of Clifford gates (e.g. gate T and hence Toffoli are not available).
I just talked to IBM (2 days ago) to ask for further details on their hardware. During this quite long meeting, they essentially explained us (me and my team) that, a part from what @MartinVesely already told you, there's a third option: the premium access (or monthly subscription), which is however very expensive (more than 10k a month, I'm not sure on the exact price but I work for a big company partnering with them and the price was higher than that). In both cases (pay-as-you-go and premium) you can only have access to the 27-qubit falcon r4 quantum computer.
In addition, they explicitly told us not to expect any speedup wrt to the simulator or any other kind of advantage, except maybe the fact that with many qubits you might not be able to use your CPU to simulate your program while the real hardware can.
Hope this helps.
The number of qubits available depends on the device that you chose. There are a couple ways to access this information, but the easiest to describe follows.
Log into your IBMQ account. On your dashboard, bottom left, there is a section called IBM Quantum Compute Resources. On the top right of this box you can 'view all'. This is the list of quantum devices available to you and each one indicates the number of qubits available when using that device. You can also locate the simulators here to see similar information!