In the UK you usually have 1-3, fewer since we left the EU. However, the number of ballots is relatively immaterial for two reasons:

Spend: In the UK, price of counting ballots is far lower than price of paying poll booth staff. It costs approximately £0.10 per person (note, not per ballot) in the UK to count ballots in a nationwide election (as opposed to about £0.35 on keeping polling booths open). It’s likely that if the number of ballots went up by a factor of 10, further efficiencies would be found, but simple ‘ballots counted per hour’ metrics imply that the marginal cost should be relatively affordable.

Difficulty: While running a fair and free election is undeniably challenging, it naively would appear to be much less work than E.g. the USPS does each day to deliver the daily 300M pieces of mail.