As someone who knows admittedly knows nothing about startup funding rounds, how many more rounds of funding can they do before an IPO? Is it effectively infinite?

Effectively infinite. Databricks is a good example. They're still private after 13 years and closed a Series L round last year. Stripe is similar.

Having been through an IPO before, it was good for employee liquidity, but bad for the culture and long-term success of the company.

I believe the canonical example is Databricks on round L

I imagine there are ways for existing investors to achieve liquidity while still raising venture funding. But an IPO is "the" liquidity event and I imagine there will be pressure from investors for that.

I also imagine that venture funding rounds have a lower ceiling than the public markets - but at these rounds I'm not so sure!

Once they reach series Z does it go back to A or do we get a new format like AA, AB ?

usually you would go through seed funding, the series a,b, and possibly a1 and b1. If you entered c or d territory it meant that you still had a chance but vc would be following you very closely. After d, you could raise money, but it would be under very unfavorable conditions

Depends on the investors if they see growth. The downside is dilution. Preferably they just want the Series I as the IPO in this case.

They cannot raise forever, SpaceX has done more rounds but the timing is most important.

Yes, whatever you like

they can do as many as they want. but at some point investors need/want to exit their positions and push for an IPO. That point is different for every company.