Addon number is relevant if you have a lot of addons that are hitting your database alot. This is what Biker told me last month and after doing some experimenting he was right.
Number itself is irrelevant. You can have 2000 addons that do almost nothing. You can have 2 addons that kill your database under high traffic.
As I said, it's more what they are doing, how those addons interact with each other ,what SQL queries they're adding and how those queries scale with traffic.
Obviously if you have a lot, there's more chance that the queries will add up and cause problems but it's not necessarily the case.