Avocados should all be somewhat self-fruitful and this gets better with size. You don’t need a Type B, but getting it should increase production.
Any avocado tree has both male (pollen shedding) and female (pollen receptive) flowers that can polinate each other, but they open on different days, like female flowers on the first day and male on the second day for Type A. The difference between Type A and B the day each type of flower opens. Opposite of Type A, Type B has male flowers on the first day and female for the second, so it matches up perfectly to pollinate Type A.
A single type will inevitably have some overlap on bloom so they will self-pollinate some. My understanding is the amount depends some on climate and variety, but larger, and more trees generally helps. Swincher (Winn) talks some about temperature affecting bloom timings here: The great Seattle cold-hardy avocado trial - #29 by swincher
At least for SoCal, I read that Reed avocado growers don’t bother interplanting with a Type B and still get good yields.
In terms cold hardy Type B varieties, I suppose Duke is one, along with Jade, poncho/pancho, and Joey to name a few. Here’s a thread on this: Type B avocado to grow with Mexicola?