when creating an azure VM, you can toggle boot diagnostics, and point to a storage account. For large deployments, is it okay just to use one boot diagnostics storage account for all your VMs? Is it the case that there is only a load placed on the boot diagnostics storage account when a VM boots? This being the case, the load on the account should be fairly light?
you can monitor the storage account as your increase the amount VM's configured to use that account.
I don't imagine it will be an issue regarding boot diagnostics, it is a very low impact feature.
One thing to consider when wanting to use a single storage account for many purposes, is how will you manage permissions to this storage account. The information stored here may not be sensitive, but just keep this in mind during your design phase.