There isn’t a technical zap limit. Depends on the receivers’ wallet type and inbound liquidity, and your outbound connections and the path’s liquidity.

Alby for example has a hard limit of 1m sats in someone’s wallet, so 999,999 is the practical zap limit to that wallet unless it’s empty. WoS has no hard cap I know of, and 2m sat zap is certainly possible.

500k zaps seem to be a reasonable upper bound at the moment, but higher zaps are possible depending on the circumstances.