Why there is a cap on open positions
Five simultaneous positions is plenty for discretionary trading and keeps exposure readable: correlated positions (for example EURUSD and GBPUSD long together) behave like one bigger trade. Count them as such when you size your risk.
Position size limits
On CFD accounts the size of each position is limited by margin and leverage; on Futures accounts each size has a maximum number of contracts. The instrument pages show the exact numbers for every account size.