If you want to spread risk, then don’t trade lots of pairs with the same currency. ie EURUSD, USDJPY, USDCHF you’d be exposed to a swing in USD affecting all 3 in the same way.
Stick to major pairs, exotics can move very quickly. GBPJPY is one of the most volatile majors, so perhaps stay away from that.
Its a rational question, especially from anyone with a background in equities, where most of us probably traded before forex. But I’m not sure there’s a realistic answer. Apart from the clear correlation between the same base currencies (e.g. USD/X, USD/Y, USD/Z) and the same counter currencies (e.g. A/JPY, B/JPY, C/JPY) most pairs move in line with their own long-term trends on most days but they all have counter-trend moves also on the same days as each other. This might be viewed as a reaction to risk rather than anything fundamental concerned with each individual currency. Net result is I think its impossible to diversify away more than a fraction of forex risk through additional forex positions.
Since you’re still learning I would pick one pair and trade it exclusively in a demo account. Taking on any more than that will slow down your rate of learning.
Stick to a few currencies - a tip I was told once is stick to your local currency and master it. For example, if you are from the US then look at USDCAD, USDJPY. Only trade 1/2 pairs to start with and master them. You don’t NEED to trade a lot of pairs to be profitable.