There is none, but here's something to look at:

Go to settings > advanced > Payment channels.

There you should see a green bar and on the left it will show your balance, on the right it shows "inbound liquidity".

This is the capacity of your wallet. When you spend, the bar gets more empty and as you receive, it gets more full.

When the bar is close to full, the channel will grow. This will typically happen as you receive some bitcoin and when you generate the invoice that will cause this, you will see a warning. It will either say "this payment will fail" with links to help you change your fee limits, or it will say "on-chain fee expected" with links to see details.

The extra fee is like an administration fee for Phoenix to allocate more of their bitcoin towards your wallet as incoming capacity, making your wallet bigger.

So all in all, I don't believe there is a limit, but maybe keeping more than a few million sats is too much anyway... If you have more than you are going to spend any time soon, consider looking into cold storage.