You can invest as much as you like in a SIPP. However, there is an annual allowance that governs how much you can claim tax relief on. For UK residents, this is £40,000 for the tax year 2019/20.*
There’s nothing to stop you contributing more than this if you want to. However, any contributions above the annual allowance will usually be subject to a tax charge.
The annual allowance covers the total contributions paid by you, or by anyone else on your behalf. Non-UK residents are not eligible for tax relief on contributions. If you have ‘adjusted income’ of over £150,000 a year, your annual allowance will be reduced by £1 for every £2 of income above £150,000, with a maximum reduction of £30,000.
* Tax laws are subject to change and depend on individual circumstances. Tax law may differ in a jurisdiction other than the UK.