how thick of a base for clay soil?

1. Depends where your frost line is i.e. where you live...

2. Yes.:yes:

central jersey

Ayuh,... If you're just Walking on it,... 4" to 6" should do...
If you're Driving on it,... 12" to 18" will do...

If you really have clay soil, you need to look into whether the soil itself needs to be compacted. We use a jumping jack to compact the heck out of clay soils.

If it were my house in my yard (central NJ), and I was redoing the pavers, I'd compact my soil with the jumping jack, then lay down some geotech fabric, then do the stone base, sand, etc. Moist, uncompacted clay soils like to move around like playdough when compressed. With the compaction and geotech fabric, I'd feel confident that such movement would be minimized.

Here's a decent link, also:

http://www.icpi.org/techspecs/index.cfm?id=31&tech=10

We add 2" to all depth specifications due to freezing etc. So 4"-6" for a patio becomes 6"-8" depth for the aggregate. You may not need that much but it wouldn't hurt.