Translation:There are seven tables.
The amount itself will never be marked with が
There are two main ways to count something; either as an adverb that modifies the verb (and goes after the thing being counted which is marked as が), or as an adjective which directly modifies the noun being counted (with the use of the particle の)
Both of these could translate to "There are seven tables"
テーブルが七つあります uses the counter 七つ as an adverb - [Tables が] [Seven] [Exist] - or "Tables exist as seven things"
七つのテーブルがあります uses the counter as an adjective attached to the noun - [Seven tables が] [Exist] - "Tables of seven quantity exist"