To discourage fraud in fiat transactions, Bisq imposes limits on trade amounts. These limits are derived from the payment method's chargeback risk, its signing status, and its age (i.e., the time since you created the payment account in Bisq).
There are no limits on the number of trades you can do—just the amount per trade.
For most bank-based payment methods like SEPA or Zelle, the maximum trade size is 0.25 BTC. For services like PerfectMoney and AliPay, it is 1 BTC.
Lower-risk payment methods start to age as soon as they're created in Bisq. When you create such a payment account in Bisq, its trade limit will be 25% of the full trade limit. After 30 days, it will increase to 50% of the full trade limit, and after 60 days it will increase to 100% of the full trade limit.
Higher-risk payment accounts must be signed by a trusted peer in order to start aging. Until this signing, they are limited to buying 0.01 BTC. The account signing doc has more details.
Note that selling limits for higher-risk payment methods are not affected by signing and follow regular account aging as described for lower-risk payment methods above.
Altcoin trades up to 2 BTC are allowed from day one—altcoins do not have chargeback risk, so account aging is not needed.