harvest upgrade/poller recommendation

[sriniji]

We are following -> https://netapp.github.io/harvest/23.11/upgrade/ for upgrade
We had the below query on harvest

1.Does Harvest upgrade to latest version require any downtime?
2.Do we lose old data after upgrade ?
3. How do we get older dashboards post upgrade ?
4.Any recommendation on how many poller should be added to single harvest (any general recommendations on scaled setup)

Can 85 pollers be added to single physical setup?
How much ram/cpu should be available on host for multiple pollers