There is an article about cricket out grounds in Wisden.
In 1965, 51 out grounds were used. Essex, Kent and Yorkshire used 6 each. Only Middlesex and Surrey held all their games on their main ground (or the MCC’s ground in Middlesex’s case).
By 2018 this had plummeted to just 9. A few more out grounds were used in 2019 because of the World Cup but it would have been back down to 9 again if this season had gone ahead as planned.
The counties played more games in the 60s in an era of three day cricket so more grounds were used partly out of necessity. However, the decline of the out ground is a real pity. They are much more enjoyable than deserted Test grounds.