May be 10 to 15
Nowadays a basic websearch can turn up some fairly good articles on wintering techniques.
For the climate you describe and for 10 to 15 colonies what is best suits you will be directly proportional to the budget you have available to put towards it. You've basically got two options - outdoor wintering, which would entail insulation and wraps. Or indoor wintering, which dedicates a climate controlled building or cellar. For 10 to 15 hives if you have an available budget of 1500$-3500$ you could come up with something that warrants the effort. Otherwise, you'll be further ahead to just cull in the fall and buy new package bees each spring. Wintering is labour intensive and expensive. Refreshing every spring also has the great benefits of not having to deal with the lingering issues or propagation of mites, beetles, etc. The longer a hive is run, the more pronounced those become. And with it escalating costs and efforts to control them.
The polystyrene type hive boxes are a good option to consider for outdoor wintering. These are pretty well already insulated by nature of the construction materials and would require minimal additional attention to prepare for cold winter. These can be used year round, as they are as good at keeping the cold out in the winter as keeping the high heat out in the summer. Use poly hive for the brood boxes, and run standard wood boxes for the honey flow. Worthwhile for 10 to 15 hives. Look into them. Though, not recommended for higher numbers (eg 100s) as they will suffer durability issues when it comes to typical equipment used when stepping up to that volume, forklifts etc.
As for indoor wintering, for that 10 to 15 count you'll be wanting a very well insulated and very dark shed of 10'x20' equipped with climate controls of fresh air fan, air conditioning, and mild heat.
Beyond the equipment, survival depends on the beekeeper preparing the hive for winter, and ultimately the strain of the bees you are running as to whether they have the robustness to survive the trials of winter. When everything else is done right, there is no better genetic selection program than winter survival.
Hope that helps with kicking off things to consider, and your research into options!