Having more than one Garden construct does not increase Botania mana generation.

[TylerSwinburne]

I added multiple "comfortable cusions" to my inner realm, but the Botania mana generation stayed the same, or possibly even went down. The time between increments in mana seemed more inconsistent when there was more than one.

[Alan19]

You can only have one garden construct active, and right clicking multiple sets the mana regeneration level to the blocks near it. The inconsistency may be from the counters for mana regeneration resetting after right clicking.