What is the easiest way to keep track of inventory?

The easiest way to keep track of inventory depends on your size, honestly tiered: under ~30 SKUs and one person — a simple spreadsheet with SKU, quantity and reorder level, updated religiously, is genuinely fine. Growing past that, the easiest reliable setup is cloud inventory software plus phone-camera barcode scanning: every receive, sale and count is a scan instead of a typed number, stock updates in real time across everyone, and sales channels sync automatically instead of being keyed in.

'Easiest' has a hidden requirement though: the easiest system is the one that stays accurate without willpower. Spreadsheets are easy to start and hard to keep true; scan-based systems cost a little setup and then protect themselves — typos disappear, updates happen at the moment of action, and reordering reminders come to you.

Start with the tier you're in; move up when surprises start costing sales.