A good question Graeme. Avocado trees if left alone will become quite large trees, so the answer to your question really depends on how much care you want to give them – are you prepared to give them a good prune yearly?
My recommendation would be that you go with what is consider to be a medium density planting and space them out approximately 6m apart. As the grow, you will need to prune them, but with just 6 trees, these wont be onerous and eventually you will be rewarded with a good harvest.
Perhaps this video will help:
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