How far apart will depend on the size you plan on growing. Smaller bonsai can be closer together than larger ones. I plant my grow beds in rows with about 18-24" between trees. Rows are about 3 feet apart to allow easy access and maintenance. I grow around 400 trees at any time so easy access is important. With just a few they can be crammed in a little closer.
Definitely allow enough space to dig them eventually. You may not spade the roots each year but will eventually need to dig down between each tree. If you don't root prune regularly you may need even more space to get in and under to cut deeper roots when you want to get the trees out.
Some pics of my grow beds for your info.
Tridents in summer. No scale but these are around head height by late summer each year. Nice and bushy because I prune regularly to promote plenty of side branches and alternate leaders.
Winter digging. Japanese maples in foreground
Pile of prunings after winter dig and prune is completed. That all goes through the chipper and back as mulch on the new season's plantings.